Method of forming amorphous silica-based coating film with low dielectric constant and thus obtained silica-based coating film

ABSTRACT

The present invention relates to an amorphous silica-based coating film with a low specific dielectric constant of 2.5 or below and the Young&#39;s modulus of 6.0 GPa or more and having excellent hydrophobic property, and to a method of forming the same. A liquid composition containing a silicon compound obtained by hydrolyzing tetraalkyl ortho silicate (TAOS) and specific alkoxysilane (AS) in the presence of tetraalkyl ammonium hydroxide (TAAOH) is prepared. The liquid composition is then applied on a substrate, heated and cured to obtain a coating film. The coating film obtained as described has a smooth surface and also has specific micropores therein.

FIELD OF THE INVENTION

The present invention relates to a method of forming on a substrate anamorphous silica-based coating film with the specific dielectricconstant of 2.5 or below and also having a high film strength andexcellent hydrophobic property with its surface being smooth or even,and further relates to an amorphous silica-based coating film with a lowdielectric constant.

BACKGROUND TECHNOLOGY

In association with the recent tendency for a higher degree ofintegration in semiconductor devices with the thickness of 0.25 micronrule or below and having multilayered wiring, as a space between themetal wires becomes narrower, impedance between the metal wiresincreases due to electrostatic induction, and therefore there is astrong concern about delay in a response speed or increase in powerconsumption. To overcome this problem, it is necessary to make as low aspossible a dielectric constant of an inter-layer insulating filmprovided between a semiconductor substrate and a metal wiring layer suchas an aluminum wiring layer or between metal wiring layers, and/or aninter-metal insulating film provided between metal wirings.

These insulating films provided for the purpose as described above aregenerally formed on a semiconductor substrate by the vapor-phase growthmethod such as the CVD method (Chemical Vapor Deposition Method) or thecoating method such as the spin coat method.

In the case of the insulating film prepared by the latest technology inthe CVD method (Refer to, for instance, the patent document 1), althoughan insulating film with the specific dielectric constant of 3 or belowcan be obtained, it is generally said that it is difficult to prepare aninsulating film with the specific dielectric constant of around 2.5 orbelow, and like in the conventional coating method, there is also adefect that the film strength of the coating film becomes lower as thespecific dielectric constant becomes lower. Further in a case of a CVDinsulating film made from polyaryl resin, fluorine-added polyimide resinand fluorine resin, or in a case of an insulating film formed with thecoating liquid, the specific dielectric constant can be lowered toaround 2, but the adhesiveness to a surface of a substrate is not good,and further also the adhesiveness to the resist material used for finemechanical processing and the like is low, and further the chemicalresistance and the resistance against oxygen plasma aredisadvantageously low.

Further with a coating film obtained by using a coating liquid forforming a silica-based coating film containing hydrolysates or partialhydrolysates of alkoxysilane and/or halogenated silane, although acoating film with the specific dielectric constant of 3 or below can beobtained, but it is difficult to achieve the specific dielectricconstant of 2.5 or below, and the adhesiveness with a coated surface israther low, which is disadvantageous.

The present inventors made strenuous efforts for solving the problemsdescribed above, and found out that, by using a) a coating liquid forforming a silica-based coating film with a low dielectric constantincluding a reaction product between alkoxysilane and/or halogenatedsilane or a hydrolysate thereof and silica fine particles (Refer to, forinstance, patent document 2), b) a coating liquid for forming asilica-based coating film with a low dielectric constant includingalkoxysilane and/or halogenated silane or a hydrolysate thereof andeasily-decomposable resin which decomposes or vaporizes at a temperatureof 500° C. or below (Refer to, for instance, patent document 3), c) acoating liquid for forming a silica-based coating film with a lowdielectric constant including polysiloxane as a reaction product betweenalkoxysilane and/or halogenated silane or a hydrolysate thereof andsilica fine particles and easily decomposable resin which decomposes orvaporizes at a temperature of 500° C. or below (Refer to, for instance,patent document 4), or d) coating liquid for forming a silica-basedcoating film With a low dielectric constant including alkoxysilaneand/or halogenated silane or a hydrolysate thereof and an organictemplate material (Refer to, for instance, patent document 5), it ispossible to form a coating film with the dielectric constant of 3 orbelow and excellent adhesiveness to a coated surface, high filmstrength, and chemical resistance such as alkaliproof characteristics,excellent cracking resistance and capable of ensuring smoothness of acoated surface, and further having high resistance against oxygen plasmaand excellent process adaptability such as etching workability.

However, the present inventors repeatedly carried out experiments forforming a silica-based coating film with a low dielectric constant onvarious types of semiconductor substrates using a coating liquids andknown methods for forming a coating film (spin coat method or othercoating methods), and found out that, although a coating film having thecharacteristics as described above can be obtained, when it is tried toform a coating film with the specific dielectric constant of 2.5 orbelow, the film strength drops and a coating film having the Young'smodulus of 6.0 GPa (giga pascal) or more strongly desired from thesemiconductor industries can hardly be obtained in the stable state.

On the other hand, researchers of the US California University haveproposed a method of forming a zeolite coating film (silica zeolitecoating film having the MFI crystalline structure) on a semiconductorsubstrate using a suspension prepared by separating and removingparticles having relative large diameters respectively from the zeolitefine particles obtained by hydrolyzing tetraethyl orthosilicate (TEOS)dissolved in ethyl alcohol in the presence of tetrapropyl ammoniumhydroxide (TPAOH). The zeolite coating film obtained by this method hasthe Young' modulus of 16 to 18 GPa, but the moisture-absorptioncharacteristics is high, so that the zeolite coating film absorbsmoisture (water vapor) contained in the air and rapidly increases thespecific dielectric constant thereof (for instance, from 2.3 to 3.9),and in that case the practicability of the zeolite coating film isdisadvantageously lost. To overcome this problem, there have beenproposed some methods including a method of keeping the specificdielectric constant of this coating film in the range from 2.1 to 2.3 bysubjecting the zeolite coating film obtained as described above tosylilation to make the surface hydrophobic (Refer to, for instance,non-patent document 1, and patent document 6).

In order to carry out the sylilation (processing by the CVD method) asdescribed above, however, in addition to the need of capital investment,complicated operations are required, so that the cost becomessubstantially high. Further as the size of zeolite particles included inthe coating film is large around 20 nm, a surface of the obtainedzeolite coating film is substantially rough, and for instance, polishingis required for smoothing the surface. Further when the zeolite coatingfilm is subjected to the processing for make it hydrophobic, only thesurface is made hydrophobic, and therefore when a wiring pattern, athrough hole or the like is formed by subjecting the coating film tofine mechanical processing such as resist application or etching,portions not having been converted to the hydrophobic state are exposedwith moisture absorbed from the portions, and as a result the specificdielectric constant of the coating film is worsened (namely madehigher), which is disadvantageous.

The present inventors made concentrated efforts for solving the problemsas described above, and found out that the problems can be solved bypreparing a coating liquid for forming a coating film having novelcompositions and properties as described below, applied the coatingliquid on a substrate, subjecting the substrate to heating step andcuring step in succession for forming an amorphous silica-based coatingfilm and completed the present invention.

-   [Patent document 1] Japanese Patent Laid-Open Publication No.    2000-349083-   [Patent document 2] Japanese Patent Laid-Open No. 1997-315812-   [Patent document 3] International Application Publication WO    00/18847-   [Patent document 4] International Application Publication WO    00/12640-   [Patent document 5] Japanese Patent Laid-Open Publication No.    2002-30249-   [Patent document 6] U.S. patent application Publication US    2000/0060364 A1-   [Non-patent document ] Advanced Material 2001, 13, No. 19, October    2, Page 1453-1466

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide, for solving theproblems as described above, a method of forming an amorphoussilica-based coating film with a low dielectric constant having thespecific dielectric constant of 2.5 or below and the film strengthexpressed by the Young's modulus of 6.0 GPa or more and capable ofensuring hydrophobic property and smoothness of a surface coatedtherewith, and also a low dielectric constant amorphous silica-basedcoating film obtained by the method described herein.

A first method of forming an amorphous silica-based coating film with alow dielectric constant according to the present invention enablesformation of silica-based coating film with a low dielectric constanthaving a high film strength and excellent hydrophobic property andcapable of ensuring smoothness of a surface coated therewith, and themethod comprises the steps of:

-   (a) preparing a liquid composition including silicon compounds    obtained by hydrolyzing tetraalkyl ortho silicate (TAOS) and    alkoxysilane (AS) expressed by the following general formula (I) in    the presence of tetraalkyl ammonium hydroxide (TAAOH):    X_(n)Si(OR)_(4-n)   (I)    wherein X indicates a hydrogen atom, a fluorine atom, or an alkyl    group, a fluorine-substituted alkyl group, an aryl group or a vinyl    group each having 1 to 8 carbon atoms; and R indicates a hydrogen    atom, or an alkyl group, an aryl group or a vinyl group each having    1 to 8 carbon atoms; n is an integral number from 0 to 3;-   (b) applying the liquid composition on a substrate;-   (c) heating the substrate at a temperature of 80 to 350° C.; and-   (d) curing the substrate at a temperature of 350 to 450° C.

A second method of forming a silica-based coating film with a lowdielectric constant according to the present invention enables formationof a silica-based coating film with a low dielectric constant having ahigh film strength and excellent hydrophobic property and capable ofensuring smoothness of a surface coated therewith, and the methodcomprises the steps of:

-   (a) preparing a liquid composition containing a silicon compound    obtained by hydrolyzing or partially hydrolyzing tetraalkyl ortho    silicate (TAOS) in the presence of tetraalkyl ammonium hydroxide    (TAAOH), mixing the reaction product with the alkoxysilane (AS)    expressed by the general formula (I) above or a hydrolysate or a    partial hydrolysate thereof, and further hydrolyzing all or a    portion of the mixture according to the necessity;-   (b) applying the liquid composition on a substrate;-   (c) heating the substrate at a temperature of 80 to 350° C.; and-   (d) curing the substrate at a temperature of 350 to 450° C.

In the method of forming the amorphous silica-based coating film, thetetraalkyl ortho silicate (TAOS) used in the preparation step (a) ispreferably tetraethyl ortho silicate (TEOS), tetramethyl ortho silicate(TMOS) or a mixture thereof.

Further the alkoxysilane used in the preparation step (a) is preferablymethytrimethoxy silane (MTMS), methyltriethoxy silane (MTES) or amixture thereof.

The tetraalkyl ammonium hydroxide (TAAOH) used in the preparation step(a) is preferably tetrapropyl ammonium hydroxide (TPAOH), tetrabutylammonium hydroxide (TBAOH) or a mixture thereof. A content of impuritiescomprising compounds of alkali metal elements such as sodium (Na) andpotassium (K) in the tetraalkyl ammonium hydroxide (TAAOH) used in thepreparation step (a) is preferably 50 ppb by weight or below onrespective element bases. Further a content of impurities comprisingcompounds of halogen group element such as bromine (Br) and chlorine(Cl) contained in the tetraalkyl ammonium hydroxide (TAAOH) used in thepreparation step (a) is preferably 1 ppm by weight or less on respectiveelement bases.

In the present invention, a molar ratio (TAOS/AS) of the tetraalkylortho silicate (TAOS) and the alkoxysilane (AS) each used in thepreparation step (a) is preferably in the range from 6/4 to 2/8 in termsof SiO₂.

Further a molar ratio (TAAOH/ (TAOS+AS)) of the tetraalkyl ammoniumhydroxide (TAAOH) and the components for forming a silica-based coatingfilm (TAOS+AS) each used in the preparation step (a) is preferably inthe range from 1/10 to 7/10 in terms of SiO₂.

Operations in the applying step (b), heating step (c) and curing step(d) are preferably performed by employing the following methodsrespectively:

-   (i) operations in the applying step (b) are performed by a spin coat    method;-   (ii) operations in the heating step (c) are performed for 1 to 10    minutes in the atmosphere of nitrogen or air; and-   (iii) operations in the curing step are performed for 10 to 90    minutes in the atmosphere of nitrogen.

On the other hand, the low dielectric constant amorphous silica-basedcoating film according to the present invention is obtained by thecoating film forming method described above, and has the specificdielectric constant of 2.5 or below and the film strength expressed bythe Young's modulus of 6.0 GPa or more. Further the coating film is anamorphous silica-based coating film not having an X-ray diffraction peakspecific to the MFI crystal structure and the like.

In addition, the average diameter of pores in the coating film ispreferably 3 nm or less, and volume percentage of micropores with thediameter of 2 nm or below against to the total volume is preferably 70%or more.

In addition, the coating film should preferably have a smooth surfacewith the surface roughness (Rms) of 1 nm or below. The surface roughnessas used herein indicates an average figure obtained by squaring valuesmeasured by an atomic force microscope (AFM).

Further preferable applications of the coating film include, forinstance, an inter-layer insulation film formed on a semiconductorsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a result of X-ray diffraction of a silica-based coatingfilm (amorphous coating film) formed on Example substrate {circle around(1)}-2, while FIG. 2 shows a result of the X-ray diffraction of asilica-based coating film (crystalline coating film) formed onComparative Example substrate {circle around (3)}. The X-ray diffractionpeaks in FIG. 2 (peaks at 2θ-8°, 9°, and 23°) indicate that the coatingfilm has the MFI crystal structure (namely, the ZSM-5 type of zeolitecoating film).

BEST MODE FOR CARRYING OUT THE INVENTION

The method of forming a silica-based coating film with a low dielectricconstant according to the present invention and the amorphoussilica-based coating film with a low dielectric constant obtainedthereby are described below.

[Method of Forming an Amorphous Silica-Based Coating Film with a LowDielectric Constant]

(a) Process for Preparing a Coating Liquid

Coating Liquid A

A first method of forming an amorphous silica-based coating film with alow dielectric constant according to the present invention uses a liquidcomposition containing a silicon compound obtained by hydrolyzingtetraalkyl ortho silicate (TAOS) and alkoxysilane (AS) expressed by thefollowing general formula (I) in the presence of tetraalkyl ammoniumhydroxide (TAAOH):X_(n)Si(OR)_(4-n)   (I)wherein X indicates a hydrogen atom, a fluorine atom, or an alkyl group,a fluorine-substituted alkyl group, an aryl group or a vinyl group eachhaving 1 to 8 carbon atoms; and R indicates a hydrogen atom, or an alkylgroup, an aryl group or a vinyl group each having 1 to 8 carbon atoms; nis an integral number from 0 to 3.

The tetraalkyl ortho silicate (TAOS) includes, but not limited to,tetramethyl ortho silicate, tetraethyl ortho silicate, tetrapropyl orthosilicate, tetraisopropyl ortho silicate, and tetrabutyl ortho silicate.It is preferable to use, of these compounds, tetraethyl ortho silicate(TEOS), tetramethyl ortho silicate (TMOS) or a mixture thereof.

The alkoxysilane (AS) includes, but not limited to,methyltrimethoxysilane, methyltriethoxysilane,methyltriisopropoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane,ethyltriisopropoxysilane, octyltrimethoxysilane, octyltriethoxysilane,vinyltrimethoxysilane, vinyltriethoxysilane, phenyltrimethoxysilane,phenyltriethoxysilane, trimethoxysilane, triethoxysilane,triisopropoxysilane, fluorotrimethoxysilane, fluorotriethoxysilane,dimethyldimethoxysilane, dimethyldiethoxysilane,dimethyldimethoxysilane, diethyldiethoxysilane, dimethoxysilane,diethoxysilane, difluorodimethoxysilane, difluorodiethoxysilane,trifluoromethyltrimethoxysilane, and trifluoromethyltriethoxysilane. Itis preferable to use, of these compounds, methytrimethoxysilane (MTMS),methyltriethoxysilane (MTES) or a mixture thereof.

The tetraalkyl ammonium hydroxide (TAAOH) includes, but not limited to,tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide,tetrapropyl ammonium hydroxide, tetrabutyl ammonium hydroxide,tetra-n-octyl ammonium hydroxide, n-hexadecyltrimethyl ammoniumhydroxide, and n-octadecyltrimethyl ammonium hydroxide. It is preferableto use, of these compounds, tetrapropyl ammonium hydroxide (TPAOH),tetrabutyl ammonium hydroxide (TBAOH), or a mixture thereof.

It is generally known that compounds of alkali metal elements such assodium (Na) and potassium (K) and the like and also compounds of halogengroup elements such as bromine (Br) and chlorine(Cl) are included at alevel from several hundreds ppm by weight to several weight % onrespective element bases as impurities in the tetraalkyl ammoniumhydroxide (TAAOH) sold on the market for general purposes.

However, when contents of the impurities comprising compounds of alkalimetal elements such as sodium (Na) and potassium (K) are over 50 ppb byweight on respective element bases, the impurities are dispersed into atransistor portion constituting a semiconductor substrate, which maysometimes causes degradation of the transistor. Further when contents ofthe impurities comprising compounds of halogen elements such as bromine(Br) or chlorine(Cl) are over 1 ppm by weight on respective elementbases, an aluminum wire or a copper wire constituting the semiconductorare corroded, which may give fatal damages to the transistor.

Further the present inventors found out that, if impurities of thealkali metal compounds are included therein by 50 ppb by weight or more,when tetraalkyl orthosilicate (TAOS) and alkoxysilane (AS) expressed bythe general formula (I) described above are hydrolyzed under thepresence of tetraalkyl ammonium hydroxide (TAAOH), the impurities act asa catalyst, and as result, the resultant silicon compound is azeolite-like crystalline silica. As a result, it was also understoodthat the silica-based coating film becomes into the zeollitic andcrystalline state, so that a surface of the silica-based coating film isnot smooth but rough.

For the reasons as described above, when commercially-availabletetraalkyl ammonium hydroxide (TAAOH) is to be used, it is necessary topreviously remove the impurities contained therein to the leveldescribed above. Namely, when the tetraalkyl ammonium hydroxide (TAAOH)is to be used in the present invention, it is preferable to previouslysubject commercially-available tetraalkyl ammonium hydroxide to thetreatment step with a cation exchange resin and then to the treatmentstep with an anion exchange resin to substantially remove the impuritiescomprising compounds of alkali metal elements such as sodium (Na) andpotassium (K) and compounds of elements belong to the halogen group suchas bromine (Br) or chlorine (Cl) for highly purifying the material.

A molar ratio (TAOS/AS) of the tetraalkyl ammonium hydroxide (TAAOH)versus alkoxysilane each used in the present invention should preferablybe in the range from 6/4 to 2/8, and more preferably in the range from5/5 to 3/7 in terms of SiO₂. When the molar ratio (TAOS/AS) is over 6/4,the hydrophobic property of the obtained silica-based coating filmbecomes lower. When the molar ratio is under 2/8, the template effectprovided by the tetraalkyl ammonium hydroxide (TAAOH) becomes lower, sothat micropores formed in the coating film are reduced, which makes itdifficult to obtain a silica-based coating film having a specificdielectric constant of 2.5 or below.

Further a molar ratio (TAAOH/(TAOS+AS)) of the tetraalkyl ammoniumhydroxide (TAAOH) versus components for forming a silica-based coatingfilm (TAOS+AS) each used in the present invention should preferably bein the range from 1/10 to 7/10, and more preferably in the range from1/10 to 6/10 in terms of SiO₂.

When the molar ratio (TAAOH/(TAOS+AS)) is under 1/10, the functionthereof as a template material is weak, so that micropores formed in thecoating film are reduced, which makes it difficult to obtain asilica-based coating film having a specific dielectric constant of 2.5or below. When the molar ratio is over 7/10, the function thereof as atemplate material is strong, so that micropores formed in the coatingfilm increase, which makes it difficult to obtain a strong silica-basedcoating film having the Young's modulus of 6.0 GPa or more. Further,when a silica-based coating film is formed on a semiconductor substrate,a portion of the material may remain in the coating film, which mayaffect the functions for a semiconductor.

A method of preparing the liquid composition, namely the coating liquidfor forming a coating film according to the present invention (coatingliquid A) is described below.

The coating liquid (coating liquid A) used in the present invention isprepared as a liquid composition containing a silicon compound orsilicon compounds which are hydrolysates of the tetraalkyl orthosilicate (TAOS) and alkoxysilane (AS) by a method comprising the stepsof:

-   (i) mixing tetraalkyl ortho silicate (TAOS) and alkoxysilane (AS)    expressed by the general formula (I) described above with an organic    solvent, and then agitating the resultant mixture until the    components are fully mixed with each other at a temperature in the    range from 10 to 30° C. at a rotating speed in the range from 100 to    200 rpm;-   (ii) adding an aqueous solution of tetraalkyl ammonium hydroxide    (TAAOH) into the mixture solution under agitation over 5 to 20    minutes, and further agitating the resultant solution for 30 to 90    minutes at a temperature in the range from 10 to 30° C. at a    rotating speed in the range from 100 to 200 rpm; and then-   (iii) heating the mixture solution at a temperature in the range    from 30 to 80° C. and agitating the solution keeping the temperature    for 1 to 72 hours at a rotating speed in the range from 100 to 200    rpm. In this case, the mixture solution comprising tetraalkyl ortho    silicate (TAOS), alkoxysilane (AS), and an organic solvent prepared    in step (i) above may slowly be added into an aqueous solution of    tetraalkyl ammonium hydroxide (TAAOH) in step (ii) over 30 to 90    minutes under the same conditions as those described above (at a    temperature in the range from 10 to 30° C. and the agitating speed    in the range from 100 to 200 rpm) instead of employing the adding    method described above (namely the method in which the aqueous    solution of TAAOH prepared in the step-   (ii) above is added into the mixture solution comprising TAOS, AS    and an organic solvent prepared in the process step (i) above) (In    other words, the methods may be categorized as a first method of    preparing a coating liquid for forming a coating film according to    the present invention).

In this step, the tetraalkyl ortho silicate (TAOS), alkoxysilane (AS),and tetraalkyl ammonium hydroxide (TAAOH) are mixed or added with themolar ratios as described above respectively in use.

The organic solvents allowable for use in the method according to thepresent invention include, but not limited to, alcohols, ketones,ethers, esters, and hydrocarbons, and more specifically alcohols such asmethanol, ethanol, propanol, and butanol; ketones such as methylethylketone, and methylisobutyl ketone; glycol ethers such as methylcellosolve, ethyl cellosolve, and propylene glycol monopropylether;glycols such as ethylene glycol, propylene glycol, and hexylene glycol;esters such as methyl acetate, ethyl acetate, methyl lactate, and ethyllactate; hydrocarbons such as hexane, cyclohexane, and octane; andaromatic hydrocarbons such as toluene, xylene, and mesitylene. It ispreferable to use, of these organic solvents, alcohols such as ethanol.

There is no specific restriction over a quantity of the organic solventto be used, but the mixing ratio by weight (organic solvent/(TAOS+AS))of the organic solvent against the components for forming a silica-basedcoating film (TAOS+AS) is preferably in the range from 1/1 to 3/1, andmore preferably in the range from 1/1 to 2.5/1.

Further the aqueous solution of tetraalkyl ammonium hydroxide (TAAOH)added into the mixture solution preferably contains tetraalkyl ammoniumhydroxide (TAAOH) by 5 to 40% by weight, and more preferably by 10 to30% by weight in distilled water or ultra pure water. The watercontained in this aqueous solution is, however, used for generatinghydrolysis of tetraalkyl ortho silicate (TAOS) and alkoxysilane (AS),and a quantity of water contained in the aqueous solution must besufficient for the hydrolysis reaction. Tetraalkyl ammonium hydroxide(TAAOH) acts as a catalyst for promoting the hydrolysis reaction, sothat there is no need for other catalyst (such as, for instance,ammonia) to be added from the outside.

The conditions for the hydrolysis reaction include a temperature in therange from 30 to 80° C., more preferably in the range from 35 to 60° C.,and the reaction should preferably be carried out under agitation over 1to 72 hours, and more preferably over 10 to 48 hours.

A numerical average molecular weight of a silicon compound (hydrolysatesof TAOS and AS) contained in the liquid composition obtained asdescribed above is in the range from 500 to 1000000, and more preferablyin the range from 1000 to 100000 in terms of polystyrene. When thenumerical average molecular weight is in the range described above, itis possible to prepare a coating liquid for forming a coating film(namely the liquid composition described above) having the long timestability and adaptability to application.

Further polysiloxane (PS) which is a reaction product between one ormore of silicon compounds selected from the group consisting ofalkoxysilane expressed by the general formula (I) below, halogenatedsilane expressed by the general formula (II) below and/or hydrolysatesthereof, and silica-based fine particles each having a diameter in therange from 5 to 50 nm may be added to the coating liquid for forming acoating film:X_(n)Si(OR)_(4-n)   (I)X_(n)SiX′_(4-n)   (II)wherein X indicates any of a hydrogen atom, a fluorine atom, or an alkylgroup, a fluorine-substituted alkyl group, an aryl group, or a vinylgroup each having 1 to 8 carbon atoms; R indicates a hydrogen atom, oran alkyl group, an aryl group, and a vinyl group each having 1 to 8carbon atoms; and X′ indicates a halogen atom. n indicates an integralnumber in the range from 0 to 3.

The silica-based fine particles can be obtained by mixing one or moretypes of alkoxysilane expressed by the general formula (I) in an organicsolvent and hydrolyzing or condensing/polymerizing the mixture solutionunder the presence of water and ammonia, and it is known that, when asilica-based coating film is formed on a substrate using a coatingliquid containing polysiloxane (PS) obtained by reacting a hydrolysateor hydrolysates of alkoxysilane and/or halogenated silane to the surfacethereof, the specific dielectric constant is 3.0 or below, a coatingfilm having relatively excellent hydrophobic property can be obtained(For the details, refer to, for instance, Japanese Patent Laid-OpenPublication No. 1997-315812).

A mixing ratio (PS/(TAOS+AS)) by weight of polysiloxane (PS) against thecomponents for forming a silica-based coating film (TAOS+AS) shouldpreferably be less than 1/3, and more preferably less than 1/4 in termsof SiO₂.

When the mixing ratio (PS/(TAOS+AS)) by weight is over 1/3, the strengthof formed silica-based coating film becomes weaker, which makes itdifficult to obtain a coating film with the Young's modulus of 6.0 GPaor more. Further the dielectric constant becomes higher, and it isdifficult to obtain a silica-based coating film with the dielectricconstant of 2.5 or below. The reason is that, as a quantity oftetraalkyl ortho silicate (TAOS) becomes smaller, the template effectprovided by tetraalkyl ammonium hydroxide (TAAOH) becomes smaller.

When the liquid composition containing the components for forming asilica-based coating film, namely a) silicon compounds which arehydrolysates of tetraalkyl ortho silicate (TAOS) and alkoxysilane (AS),or b) silicon compounds which are hydrolysates of tetraalkyl orthosilicate (TAOS) and alkoxysilane (AS) and polysiloxane (PS) obtained asdescribed above is used as a coating liquid for forming a coating film,the coating film should preferably contain the components for forming asilica-based coating film (silicon compounds, or silicon compounds andPS) by 2 to 40% by weight, and more preferably by 5 to 20% by weight interms of SiO₂.

When the content is over 40% by weight, the long period stability of thecoating liquid becomes worse. When the content is under 2% by weight, itbecomes difficult to form a homogeneous coating film.

In the coating liquid A according to the present invention, the liquidcomposition containing components for forming a silica-based coatingfilm obtained by the method described above may be used as it is as acoating liquid for forming a coating film, but it is preferable tosubject the liquid composition, employing a rotary evaporator or thelike, to a process for solvent-substituting the organic solventcomponent contained in the liquid composition with an organic solventselected from the group consisting of propylene glycol monopropylether(PGC), propylene glycol monomethylether (PGME), and propylene glycolmonoethylether acetate (PGMEA) and the like, and thereafter foradjusting the concentration of the components for forming a silica-basedcoating film to the level as described above. In this process forsolvent substitution, the organic solvent, water, and further alcoholsproduced through hydrolysis of alkoxysilane (AS) are separated andremoved, but the liquid composition obtained after the substitutionprocess should still contain the organic solvent and water containedbefore the process in the range from 0.1 to 40% by weight, and morepreferably in the range from 0.1 to 30 weight % against the total weightof the liquid contents respectively.

When the conditions are satisfied, it is possible to obtain a coatingfilm for forming an amorphous silica-based coating film with a lowdielectric constant and also having a high film strength and excellenthydrophobic property with its surface being smooth or even.

Coating Liquid B

In a second method of forming a low dielectric constant silica-basedcoating liquid, as a liquid composition for forming a coating film, aliquid composition containing silicon compounds obtained by hydrolyzingor partially hydrolyzing tetraalkyl ortho silicate (TAOS) under thepresence of tetraalkyl ammonium hydroxide (TAAOH), then mixing theresultant product in alkoxysilane (AS) expressed by the general formula(I) below or a product obtained by hydrolyzing or partially hydrolyzingthe material, and further hydrolyzing or partially hydrolyzing all or aportion of the mixture is used:X_(n)Si(OR)_(4-n)   (I)wherein X indicates any of a hydrogen atom, a fluorine atom, or an alkylgroup, a fluorine-substituted alkyl group, an aryl group, or a vinylgroup each having 1 to 8 carbon atoms; and R indicates a hydrogen atom,or an alkyl group, an aryl group, and a vinyl group each having 1 to 8carbon atoms; n indicates an integral number in the range from 0 to 3.

The tetraalkyl ortho silicate (TAOS), alkoxysilane (AS), and tetraalkylammonium hydroxide (TAAOH) used in this step may be the same as thoseused for preparing the coating liquid A described above.

It is necessary to subject the tetraalkyl ammonium hydroxide (TAAOH)commercially available, like in preparation of the first coating liquidA, to the processing step with the cation exchange resin and also to theprocessing step with the anion exchange resin to substantially removethe impurities containing therein and comprising compounds of alkalimetal elements such as sodium (Na) and potassium (K) and those ofhalogen group elements such as bromine (Br) and chlorine (Cl). Namely itis necessary to reduce a content of the impurities comprising compoundsof alkali metal elements such as sodium (Na) and potassium (K) to 50 ppbby weight on each element basis, and also to reduce a content of halogengroup elements such as bromine (Br) and chlorine (Cl) to 1 ppm by weighton each element basis.

The liquid composition (coating liquid B) is prepared by hydrolyzing orpartially hydrolyzing tetraalkyl ortho silicate (TAOS), mixingalkoxysilane (AS) or a product obtained by hydrolyzing or partiallyhydrolyzing the alkoxysilane (AS) in the product obtained above, andfurther hydrolyzing or partially hydrolyzing all or a portion of themixture according to the necessity.

When the alkoxysilane (AS) is previously hydrolyzed (or partiallyhydrolyzed) and then mixed therein, the operation should preferably becarried out under the presence of tetraalkyl ammonium hydroxide (TAAOH)like in the case of hydrolyzing (or partially hydrolyzing) tetraalkylortho silicate (TAOS).

In this case, the tetraalkyl ammonium hydroxide (TAAOH) shouldpreferably be added, like in preparation of the coating liquid A, sothat the molar ratio (TAAOH/TAOS and TAAOH/AS) against tetraalkyl orthosilicate (TAOS) and alkoxysilane (AS) is in the range from 1/10 to 7/10,more preferably in the range from 1/10 to 6/10 respectively. (Therefore,the molar ratio (TAAOH/(TAOS+AS)) of the tetraalkyl ammonium hydroxide(TAAOH) against the components for forming a silica-based coating film(TAOS +AS) is in the range from 1/10 to 7/10, and more preferably in therange from 1/10 to 6/10 in terms of SiO₂ like in the case of the coatingliquid A).

Further when the components are mixed, like in preparation of thecoating liquid A, the molar ratio (TAOS/AS) of the tetraalkyl orthosilicate (TAOS) versus the alkoxysilane (AS) should preferably be in therange from 6/4 to 2/8, and more preferably in the range from 5/5 to 3/7in terms of SiO₂.

Next a method of preparing the coating liquid for forming a coating film(coating liquid B) according to the present invention is describedbelow.

The coating liquid for forming a coating film (coating liquid B) used inthe present invention is a liquid composition containing a siliconcompound which is a hydrolysate of the tetraalkyl ortho silicate (TAOS)and alkoxysilane (AS) prepared by a method comprising the steps of:

-   (i) mixing tetraalkyl ortho silicate (TAOS) with an organic solvent,    and agitating the mixture at a temperature from 10 to 30° C. and at    a rotating speed of 100 to 200 rpm until the components are fully    mixed with each other;-   (ii) adding an aqueous solution of tetraalkyl ammonium hydroxide    (TAAOH) into the mixture solution being agitated over 5 to 20    minutes, and further agitating the resultant mixture for 30 to 90    minutes at a temperature from 10 to 30° C. and at a rotating speed    of 100 to 200 rpm;-   (iii) heating the mixture to a temperature from 30 to 80° C.,    agitating the mixture keeping the temperature for 0.5 to 72 hours at    a rotating speed of 100 to 200 rpm to prepare a mixture solution    containing a product obtained by hydrolyzing and/or partially    hydrolyzing the tetraalkyl ortho silicate (TAOS);-   (iv) further mixing the alkoxysilane (AS) expressed by the general    formula (I) or a mixture thereof with an organic solvent in the    mixture solution obtained in the step (iii) above, and agitating the    resultant mixture at a temperature from 10 to 30° C. and at a    rotating speed of 100 to 200 rpm until the components are fully    mixed with each other;-   (v) adding an aqueous solution of tetraalkyl ammonium hydroxide    (TAAOH) into the mixture solution above being agitated over 5 to 20    minutes and further agitating the resultant mixture solution for 30    to 90 minutes at a temperature from 10 to 30° C. and at a rotating    speed of 100 to 200 rpm; and-   (vi) heating the mixture solution obtained in the step (v) above to    a temperature in the range from 30 to 80° C. and agitating the    solution keeping the temperature level for 10 to 30 hours at a    rotating speed of 100 to 200 rpm. In this case, in place of the    adding method described above (namely the method of adding an    aqueous solution of TAAOH obtained in the step (ii) into a mixture    solution of TAOS and an organic solution prepared in the step (i)),    the mixture solution comprising tetraalkyl ortho silicate (TAOS) and    an organic solvent prepared in the step (i) may slowly be added into    the aqueous solution of the tetraalkyl ammonium hydroxide (TAAOH) in    the step (ii) over 30 to 90 minutes under the same conditions    described above (at a temperature from 10 to 30° C. and at a    rotating speed from 100 to 200 rpm). (Namely either one of the    methods may be employed as a second method for preparing the coating    liquid for forming a coating film according to the present    invention.)

Further the coating liquid for forming a coating film (coating liquid B)used in the present invention is a liquid composition including asilicon compound which is hydrolysate of tetraalkyl ortho silicate(TAOS) and alkoxysilane (AS), and is prepared by a method comprising thesteps of:

-   (i) mixing tetraalkyl ortho silicate (TAOS) in an organic solvent,    and agitating the mixture solution at a temperature of 10 to 30° C.    and at a rotating speed of 100 to 200 rpm until the components are    mixed with each other;-   (ii) adding an aqueous solution of tetraalkyl ammonium hydroxide    (TAAOH) into the mixture solution being agitated, and further    agitating the resultant solution for 30 to 90 minutes at a    temperature of 10 to 30° C. and at a rotating speed of 100 to 200    rpm;-   (iii) heating the resultant solution to a temperature of 30 to 80°    C., agitating the mixture solution keeping the temperature for 0.5    to 72 hours at a rotating speed of 100 to 200 rpm to prepare a    mixture solution containing a hydrolysate and/or partial hydrolysate    of the tetraalkyl ortho silicate (TAOS);-   (iv) further mixing the alkoxysilane (AS) expressed by the general    formula (I) in an organic solvent, and agitating the mixture    solution at a temperature from 10 to 30° C. and at a rotating speed    of 100 to 200 rpm until the components are fully mixed with each    other;-   (v) then adding tetraalkyl ammonium hydroxide (TAAOH) into the    mixture solution being agitated over 5 to 20 minutes, and agitating    the resultant mixture solution for 30 to 90 minutes at a temperature    from 10 to 30° C. and at a rotating speed of 100 to 200 rpm;-   (vi) heating the mixture solution to a temperature of 30 to 80° C.,    mixing the mixture solution keeping the temperature for 0.5 to 72    hours at a speed of 100 to 200 rpm to prepare a mixture solution    containing a hydrolysate and/or a partial hydrolysate of the    alkoxysilane (AS);-   (vii) then mixing the mixture solution prepared in the step (iii)    above with the mixture solution obtained in the step (vi), and    agitating the resultant mixture solution at a temperature from 10 to    30° C. and a rotating speed of 100 to 200 rpm until the components    are fully mixed with each other; and-   (viii) further heating the solution obtained in the step (vii) above    to a temperature to 30 to 80° C. according to the necessity, and    then agitating the mixture solution keeping the temperature for 10    to 30 hours at a rotating speed of 100 to 200 rpm.

In this case, in place of the adding method described above (namely themethod of adding the aqueous solution of TAAOH obtained in the step (ii)into the mixture solution comprising TAOS and an organic solventprepared in the step (i) above, and also adding the aqueous solution ofthe TAAOH obtained in the step (v) into the mixture solution comprisingthe AS and an organic solvent prepared in the step (v)), it is possibleto employ a method comprising the steps of gradually adding the mixturesolution comprising tetraalkyl ortho silicate (TAOS) and an organicsolvent prepared in the step (i) above into the aqueous solution oftetraalkyl ammonium hydroxide (TAAOH) prepared in the step (ii) over 30to 90 minutes under the same conditions as those described above (at atemperature of 10 to 30° C. and at a rotating speed of 1 00 to 200 rpm),and further adding the mixture solution comprising alkoxysilane (AS) andan organic solvent prepared in the step (iv) into the aqueous solutionof tetraalkyl ammonium hydroxide (TAAOH) prepared in the step (v) aboveover 30 to 90 minutes under the same conditions as those described above(at a temperature of 10 to 30° C. and at a rotating speed of 100 to 200rpm). (Namely, each of these methods may be employed as a third methodfor preparing the coating liquid for forming a coating film according tothe present invention).

In this step, the tetraalkyl ortho silicate (TAOS), alkoxysilane (AS)and tetraalkyl ammonium hydroxide (TAAOH) are mixed with each other oradded to satisfy the molar ratios described above respectively.

As the organic solvent, the same one as that as an example available forpreparation of the coating liquid A may be used. The organic solventsmixed in tetraalkyl ortho silicate (TAOS) and in alkoxysilane (AS) maybe different on the condition that the two organic solvents belong tothe same type (such as, for instance, alcohols), but are preferably thesame ones.

There is no specific restriction over a quantity of the organic solventto be used, but the mixing ratio thereof (organic solvent/(TAOS+AS))against the components for forming a silica-based coating film (TAOS andAS) should preferably be in the range from 1/1 to 3/1, and morepreferably in the range from 1/1 to 2.5/1. Therefore, the mixing ratioby weight (organic solvent/(TAOS+AS)) of the organic solvent with thecomponents mixed therein versus the components for forming asilica-based coating film (TAOS +AS) should preferably be in the rangefrom 1/1 to 3/1, and more preferably in the range from 1/1 to 2.5/1 likein the case of coating liquid A.

Further the aqueous solution of tetraalkyl ammonium hydroxide (TAAOH) tobe added into the mixture organic solvent should preferably containtetraalkyl ammonium hydroxide (TAAOH) by 5 to 40% by weight, and morepreferably by 10 to 30% by weight in distilled water or ultra pure waterlike in the case of the coating liquid A.

The hydrolysis described above should preferably be performed, like inthe case of the coating liquid A, by agitation for 0.5 to 72 hours andmore preferably for 10 to 48 hours at a temperature of 30 to 80° C., andmore preferably at a temperature of 35 to 60° C. In the secondpreparation method and third preparation method described above, aperiod of time spent for hydrolyzing only the tetraalkyl ortho silicate(TAOS) or alkoxysilane (AS) may be short, but a period of time requiredto completely hydrolyze the components (such as 10 to 30 hours) shouldpreferably be spent for this reaction in the next step.

A numeric average molecular weight of silicon compounds (hydrolysates ofTAOS and AS) contained in the liquid composition obtained as describedabove are, like in the case of the coating liquid A, in the range from500 to 1000000, and more preferably in the range from 1000 to 100000 interms of polystyrene.

Further polysiloxane (PS) which is a reaction product between one ormore silicon compounds selected from the group consisting of thealkoxysilane expressed by the general formula (I) and halogenated silaneexpressed by the general formula (II) below and/or hydrolysates thereof,and silica-based fine particles each having particle size from 5 to 50nm may be contained in the coating liquid for forming a coating filmaccording to the necessity like in the case of the coating liquid A:X_(n)Si(OR)_(4-n)   (I)X_(n)SiX′_(4-n)   (II)wherein X indicates any of a hydrogen atom, a fluorine atom, or an alkylgroup, a fluorine-substituted alkyl group, an aryl group, or a vinylgroup each having 1 to 8 carbon atoms; R indicates a hydrogen atom, oran alkyl group, an aryl group, and a vinyl group each having 1 to 8carbon atoms; and X′ indicates a halogen atom; n indicates an integralnumber in the range from 0 to 3.

However, a content of the polysiloxane (PS) as expressed by a weightmixing ratio (PS/(TAOS+AS)) against the components for forming asilica-based coating film (TAOS+AS) should preferably be less than 1/3,and more preferably less than 1/4 in terms of SiO₂.

When the liquid composition containing the components for forming asilica-based coating film, namely a) silicon compounds which arehydrolysates of tetraalkyl ortho silicate (TAOS) and alkoxysilane (AS),or b) silicon compounds which are hydrolysates of tetraalkyl orthosilicate (TAOS) and alkoxysilane (AS) and polysiloxane (PS) is used as acoating liquid for forming a coating film, like in the case of thecoating liquid A, the coating liquid should preferably contain thecomponents for forming a silica-based coating film (silicon compounds,or silicon compounds and PS) by 2 to 40% by weight, and more preferablyby 5 to 20% by weight in terms of SiO₂.

In the coating liquid B according to the present invention, like in thecase of the coating liquid A, the liquid composition containing thecomponents for forming a silica-based coating film obtained by themethod described above may be used as a coating liquid for forming acoating film as it is, but it is preferable to subject the coatingliquid to the process for solvent-substituting the organic solventcomponent contained in the liquid composition with an organic solventselected from the group consisting of propylene glycol monopropylether(PGP), propylene glycol monomethyl ether (PGME), propylene glycolmonoethyl ether acetate (PGMEA) and the like with a rotary evaporator,and then to adjust concentrations of the components for forming asilica-based coating film to the level as described above respectivelyfor use. In this process for solvent substitution, the organic solvent,water, and further alcohols produced through hydrolysis of alkoxysilane(AS) contained in the liquid composition are separated and removed, butthe liquid composition obtained after the substitution process shouldstill contain the organic solvent and water contained before the processpreferably in the range from 0.1 to 40% by weight, and more preferablyin the range from 0.1 to 30 weight % against the total weight of theliquid contents respectively.

When the conditions are satisfied, it is possible to obtain a coatingfilm for forming an amorphous silica-based coating film with a lowdielectric constant and also having a high film strength and excellenthydrophobic property with its surface being smooth or even.

(b) Applying Step

Generally, such methods as the spin coat method, dip coat method, rollcoat method, and transcription method are employed for applying thecoating liquid as described above, and also in the present invention, anamorphous silica-based coating film having a low dielectric constant canbe formed using any of the methods based on the conventional technologyas described above. Of these methods, when a coating liquid for forminga coating film is applied, for instance, on a semiconductor substrate,the spin coat method is most preferable because of the homogeneity inits film thickness, low dust generating property, and the like.Therefore, in the present invention, the coating method based on thespin coat method is preferably employed, but when the coating liquid forforming a coating film is applied, for instance, on a semiconductorsubstrate with a larger diameter, the transcription method or the likemay be employed.

(c) Heating Step

The coating film applied on a substrate is heated at a temperature of 80to 350° C.

When the heating step is carried out at a temperature over 350° C., anorganic solvent contained in the applied coating film rapidlyevaporates, and pores or voids each having a relatively large diameterare formed in the coating film, so that the strength of the coating filmmay largely drop. For the reason as described above, the heating step ispreferably carried out raising the temperature step by step according tothe necessity in the range from 80 to 350° C. Further when this heatingsteping is carried out at a temperature lower than 80° C., the organicsolvent contained in the coating film little evaporates, and remains inthe coating film, which spoils the purpose of heating step, and also thethickness of a formed coating film becomes inhomogeneous.

Although a required period of time for this heating step variesaccording to such factors as a required film thickness, generally theheating step is preferably carried out for 1 to 10 minutes, andpreferably for 2 to 5 minutes.

Further the heating step may be carried out in a nitrogen gas atmosphereas an inert gas or in the air. However, in the present invention, theheating step is preferably carried out in the air. As this heating stepis carried out for a short period of time at a temperature of 350° C. orbelow which is relatively low, even if this heating step is carried outin the air containing oxygen of about 21% by volume, damages such asoxidization of metals are not given to the metal wiring provided on thesemiconductor substrate. Further as a small quantity of oxygen may befetched into the coating film, a silica-based coating film with a higherdegree of —Si—O—Si— bridging is generated during the curing step (c)described hereinafter, which makes higher the possibility of formationof a silica-based coating film with excellent moisture-absorptionresisting characteristics and high strength and also having a lowdielectric constant.

When the heating step as described above is carried out, the organicsolvent contained in the applied coating liquid evaporates, and also thetetraalkyl ammonium hydroxide (TAAOH) contained in the coating film isdecomposed or separated, which promotes polymerization and curing of thecomponents for forming a silica-based coating film which are solidcomponents, and also the molten viscosity of a polymer becomes lowerduring the heating step, so that the reflowing property of the coatingfilm increases, and therefore the smoothness of an obtained coating filmis improved. The heating step should preferably be carried out byplacing the substrate obtained in the applying step described above on ahot plate of a single wafer system.

(d) Curing Step

Next, the coating film having been subjected to the heating step iscured at a temperature from 350 to 450° C. in the atmosphere of inertgas. It is preferable to use a nitrogen gas as the inert gas, andfurther, if required, an inert gas containing a small volume of oxygen(for instance by about 500 to about 10000 volume ppm) produced by addingan oxygen gas or air to the inert gas above may be used for the samepurpose (as described, for instance, in International Patent ApplicationPublication No. WO 01/48806 A1).

As for the curing step, a temperature in the range from 350 to 450° C.is preferably employed for obtaining an amorphous silica-based coatingfilm with excellent moisture absorption characteristics (hydrophobicproperty) and high strength and having a low dielectric constant,although the required temperature may change according to a type and aquantity of tetraalkyl ammonium hydroxide (TAAOH) used for preparationof the coating liquid for forming a coating film, or to characteristicsof silicon compounds (namely components for forming a silica-basedcoating film) contained in the coating liquid.

When the curing temperature is under 350° C., bridging hardly occurs inprecursors of components for forming a silica-based coating film, sothat a coating film having sufficient strength can hardly be obtained,and when the curing temperature is over 450° C., aluminum wiring orcopper wiring arranged on a semiconductor substrate may be oxidized ormelted, which may give fatal damages to the wiring layer.

The curing step is preferably carried out over 5 to 90 minutes, and morepreferably for 10 to 60 minutes, although the required period of timevaries according to a type of the coating liquid for forming a coatingfilm or a thickness of a coating film to be formed. Further the heatingstep should preferably be carried out by placing the substrate obtainedin the applying step described above on a hot plate of a single wafersystem.

The thickness of a silica-based coating film obtained as described aboveis generally in the range from 100 to 600 nm, for instance, on a siliconsubstrate (silicon wafer) in a semiconductor device, although therequired thickness varies according to a semiconductor substrate onwhich a coating film is to be formed or the purpose, but is generally inthe range from 100 to 1000 nm between wiring layers in a multilayeredwiring layer.

[Amorphous Silica-Based Coating Film with a Low Dielectric Constant]

A coating film formed by using the coating liquid according to thepresent invention has a specific dielectric constant of 2.5 or below andYoung's modulus of 6.0 GPa or more. Further with the coating liquidaccording to the present invention, a silica-based coating film withpores having the average diameter of 3 nm or below and also with acontent of micropores each with the diameter of 2 nm or below of 70% ormore can easily be formed. The physical properties described above areimportant for forming the coating film having high strength and a lowdielectric constant. For the reasons as described above, with thepresent invention, it is possible to form a silica-based coating filmsatisfying the requirements from the semiconductor manufacturers inrecent years.

Further with the coating liquid according to the present invention, itis possible to easily form a silica-based coating film having thesurface roughness (Rms) of 1 nm or below and also having a smoothsurface. This surface roughness is defined as a square average roughnessof values measured with an atomic force microscope (AFM). With thecharacteristics, it is not always required to perform the complicatedpolishing process for flattening a surface of a coating film formed on asubstrate, so that it is possible to overcome the defects of thezeolitic coating film as described above.

In addition, the silica-based coating film formed with the coatingliquid according to the present invention has the excellent hydrophobicproperty (moisture absorption resisting characteristics), and thereforeeven if the coating film is left in the air atmosphere containingsaturated water vapor, worsening the specific dielectric constant(namely increase in the specific dielectric constant) like that in thezeolitic film never occurs. Further there is no need for subjecting asurface of the coating film to silylation as required for a zeoliticfilm. The silica-based coating film obtained by using the coating liquidaccording to the present invention is an amorphous silica-based coatingfilm not having an X-ray diffraction peak such as that observed in theMFI crystal structure of a zeolitic coating film.

Further, the coating liquid according to the present invention is usedfor forming a coating film on a semiconductor substrate, between wiringlayers in a multilayered wiring layer, on a substrate with an elementsurface and/or a PN junction section provided thereon, or betweenmultilayered wiring layers provided on each of the substrates asdescribed above. Of these, the coating liquid according to the presentinvention is advantageously used for forming an inter-layer insulationfilm on a semiconductor substrate and the like.

With the method according to the present invention, an amorphoussilica-based coating film having a low dielectric constant of 2.5 orless as well as having the high strength expressed by the Young'smodulus of 6.0 GPa or more and excellent moisture absorption resistingcharacteristics (hydrophobic property) can easily be formed on asubstrate without subjecting a surface of the coating film to silylationor the like. Further with the coating liquid according to the presentinvention, an amorphous silica-based coating film with a low dielectricconstant and also having a smooth surface with the surface roughness(Rms) of 1 nm or below can be formed on a substrate without subjecting asurface of the coating film to the polishing process or the like.

Further, the silica-based coating film obtained by using the coatingliquid according to the present invention has, in addition to theproperties as described above, the excellent adhesiveness to a surfacefor forming a coating film thereon such as a semiconductor substrate,chemical resistance such as alkaliproof, and cracking resistance, andfurthermore has excellent process adaptability such as oxygen plasmaresistance or workability by etching. Namely, in the silica-basedcoating film formed with the coating liquid according to the presentinvention, in addition to the effects and advantages achieved by theinventions applied for patents in the past by the present inventors,there are provided such effects and advantages as the low specificdielectric constant of 2.5 or below, high film strength expressed by theYoung's modulus of 6.0 GPa or more, and excellent moisture absorptionresisting characteristics (hydrophobic property).

Examples of the present invention are described in detail below, but thepresent invention are not limited to the examples.

EXAMPLE 1

300 g cation exchange resin powder (WK-40, produced by MitsubishiChemicals Co.) was added to 1 kg aqueous solution containing 40% byweight of tetrapropyl ammonium hydroxide (TPAOH, produced by LionCorp.), and the resultant mixture solution was agitated for one hour atthe room temperature and at a rotating speed of 100 rpm, and then theadded cation exchange resin powder was removed by filtration. Then 2100g anion exchange resin powder (SAT-10, produced by Mitsubishi ChemicalsCo.) was added, and the resultant mixture was agitated for one hour at arotating speed of 100 rpm, and the added anion exchange resin powder wasremoved by filtration.

Ultra pure water was added to the obtained aqueous solution oftetrapropyl ammonium hydroxide (TPAOH) to adjust the concentration to10% by weight, and quantities of compounds of alkali metal elements suchas sodium (Na) and potassium (K) and also compounds of halogen groupelements such as bromine (Br) and chlorine (Cl) contained as impuritiesin the aqueous solution were measured by the atomic absorptionspectrometry method (AAS method, with a polarized Zeeman atomicabsorption photometer Z-5710 produced by Hitachi Inc.) and ionchromatography method (with 2020i produced by DIONEX) respectively.

Further ultra pure water was added an aqueous solution of thetetrapropyl ammonium hydroxide (commercially procurable) not having beensubjected to the ion exchange processes described above to adjust theconcentration to 10% by weight, and contents of impurities containedtherein were measured as described above.

As a result, contents of the impurities contained in the aqueoussolution not having been subjected to the ion exchange processes were 50ppm by weight for sodium, 2500 ppm by weight for potassium, 2250 ppm byweight for bromine, and 13 ppm by weight for chlorine as converted toeach element respectively, but those of the impurities contained in theaqueous solution having been subjected to the ion exchange processeswere 10 ppb by weight or below for sodium (lower limit for detection),10 ppb by weight or below for potassium (lower limit for detection), 1ppm by weight or below for bromine, and 1 ppm by weight or below forchlorine as converted to each element respectively. Namely the aqueoussolution of tetrapropyl ammonium hydroxide (commercially availableproduct) could be highly purified to the minimum content level ofimpurities allowable in the present invention.

Then, tetraethyl ortho silicate (TEOS, produced by Tama ChemicalIndustry Corp.), methyltrimethoxy silane (MTMS, produced by Shin'etsuKagaku K. K), and ethanol with the concentration of 99.5% by weight(ETOH, produced by Wako Jun'yaku K. K) were mixed at mixing ratios asshown in Table 1, and the mixture solution was kept at a temperature of20° C. and agitated for 30 minutes at a rotating speed of 150 rpm.

The highly purified aqueous solution of tetrapropyl ammonium hydroxide(containing TPAOH by 10% by weight) was added over 10 minutes into themixture solution at the mixing ratio as shown in Table 1, and theresultant mixture solution was kept at a temperature of 20° C. andagitated for 1 hour at a rotating speed of 200 rpm. Then the mixturesolution was heated to a temperature of 50° C. and agitated keeping thetemperature at the level above for 20 hours at a rotating speed of 200rpm to hydrolyze the components for forming a silica-based coating film(TEOS and MTMS).

Then mixture solution containing hydrolysates of the components forforming a silica-based coating film was subjected to the solventsubstitution process for substituting ethanol (organic solvent)contained therein with propylene glycol monopropyl ether (PGP, producedby Nihon Emulsion Co.) using a rotary evaporator (R-114 produced byShibata Kagaku Co.), and then concentrations of silicon compoundscomprising hydrolysates of tetraethyl ortho silicate (TEOS) andmethyltrimethoxy silane (MTMS) were adjusted to obtain liquidcompositions each containing the compounds by 12% by weight in terms ofSiO₂ (Examples of coating liquid {circle around (1)}-1 to {circle around(1)}-8). The requirements for preparation of the liquid compositions(coating liquid for forming a coating film) are as shown in Table 1.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was dripped on silicon wafer substrates (semiconductorsubstrates) each with the size of 8×8 square inches using the spin coatmethod known in the conventional technology (ACT-8 produced by TokyoElectron Co.) and was applied thereon for 20 seconds at a rotating speedof 2000 rpm.

Then the substrates were placed on a hot plate of a single wafer systemand were heated for 3 minutes at a temperature of 150° C. in thenitrogen atmosphere. Some components of the coating film such as theorganic solvent (PGP) evaporated during the heating step, and theevaporated components were eliminated to the outside.

Further in the state where the substrates were placed on the hot plateof a single wafer system, the substrates were cured for 30 minutes at atemperature of 400° C in the nitrogen atmosphere. Then the substrates(Example substrates ({circle around (1)}-1 to {circle around (1)}-8)were cooled down to around the room temperature, and taken out from thesystem.

The film thickness of the silica-based coating film formed on each ofthe substrate obtained as described above (measured with aspectro-ellipsometer ESVG produced by SOPRA) was about 500 nm.

The silica-based coating film formed on the substrate was measured forthe following matters: (i) specific dielectric constant (with themercury probe method with the frequency of 1 MHz, SSM 495 produced bySolid State Measurements), (ii)change rate in moisture absorption by thecoating film before and after irradiation of an oxygen plasma (with theTDS method, Thermal Desorption Mass-Spectroscopy, EDM-1000 produced byDenshi Kagaku Co.), (iii) film strength (Young's modulus,nano-indentation method, Nano-indenter XP produced by MTS SystemsCorp.), (iv) surface roughness (Rms, AFM method), (v) pore distribution(average diameter of pores and volume percentage of micropores each withthe diameter of 2 nm or below against the total volume: nitrogenabsorption method), and (vi) X-ray diffraction peak (determined for ancrystalline coating film or an amorphous coating film: X-ray diffractionmethod). The processors and measuring devices or apparatuses used inExample 1 were also used in Examples 2 to 8 and Comparative Examples 1to 5 described hereinafter.

The result of the measurement is shown in Table 5. Further, the resultof X-ray diffraction of the silica-based coating film formed on Examplesubstrate {circle around (1)}-2 is shown in FIG. 1.

EXAMPLE 2

60.8 g tetramethyl ortho silicate (TMOS, produced by Tama ChemicalIndustry Corp.), 127.3 g methyl trimethoxy silane (MTMS, produced byShin'etsu Kagaku Kogyo Corp.) and 513.6 g ethanol with the concentrationof 99.5% by weight (ETOH, produced by Wako Jun'yaku Corp.) were mixedwith each other, and this mixture solution was kept at a temperature of20° C. and agitated for 30 minutes at a rotating speed of 150 rpm.

293.3 g of highly purified aqueous solution of the tetrapropyl ammoniumhydroxide (containing TPAOH by 10% by weight) was added into the mixturesolution over 10 minutes, and the resultant mixture solution wasagitated for 1 hour at a temperature of 20° C. and at a rotating speedof 150 rpm. Then the resultant solution was heated to 50° C., and wasagitated for 20 hours at the same temperature and at a rotating speed of200 rpm to hydrolyze the components for forming a silica-based coatingfilm (TMOS and MTMS).

Further, like in Example 1, the mixture solution was subjected to thesolvent substitution process to substitute ethanol (organic solvent) inthe mixture solution containing hydrolysates of the components forforming a silica-based coating film with propylene glycol monopropylether (PGP) using a rotary evaporator, and then concentrations ofsilicon compounds comprising hydrolysates of tetramethyl ortho silicate(TMOS) and methyl trimethoxy silane (MTMS) were adjusted to obtainliquid compounds each containing the compounds by 12% by weight in termsof SiO₂ respectively (Example of coating liquids {circle around (2)}).The requirements for preparation of the liquid composition (coatingliquid for forming a coating film) are as shown in Table 1.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon wafer substrate (semiconductorsubstrate) with the size of 8×8 square inches using the spin coat methodunder the same conditions as those employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrates (Example substrate {circle around (2)}) wascooled down to a temperature around the room temperature, and taken outfrom the system.

The film thickness of the silica-based coating film formed on asubstrate obtained as described above was about 500 nm.

Then, like in Example 1, the silica-based coating film formed on asubstrate was measured for the specific dielectric constant, a changerate in moisture absorption by the coating film before and afterirradiation of an oxygen plasma, film strength, surface roughness, poredistribution (average diameter of pores and volume percentage ofmicropores each with the diameter of 2 nm or below against the totalvolume), and X-ray diffraction peaks (determined for an crystallinecoating film or an amorphous coating film).

The result is shown in Table 5.

EXAMPLE 3

85.7 g tetraethyl ortho silicate (TEOS, produced by Tama ChemicalIndustry Corp.), 166.2 g methyl trimethoxy silane (MTMS, produced byShin'etsu Kagaku Kogyo Corp.) and 449.8 g ethanol with the concentrationof 99.5% by weight (ETOH, produced by Wako Jun'yaku Corp.) were mixedwith each other, and this mixture solution was kept at a temperature of20° C. and agitated for 30 minutes at a rotating speed of 150 rpm.

298.3 g of highly purified aqueous solution of the tetrapropyl ammoniumhydroxide (containing TPAOH by 10% by weight) was added into the mixturesolution over 10 minutes, and the resultant mixture solution wasagitated for 1 hour at a temperature of 20° C. and at a rotating speedof 150 rpm. Then the resultant solution was heated to 50° C., and wasagitated for 20 hours at the same temperature and at a rotating speed of200 rpm to hydrolyze the components for forming a silica-based coatingfilm (TEOS and MTES).

Then, like in Example 1, the mixture solution was subjected to thesolvent substitution process to substitute ethanol (organic solvent) inthe mixture solution containing hydrolysates of the components forforming a silica-based coating film with propylene glycol monopropylether (PGP) using a rotary evaporator, and then concentrations ofsilicon compounds comprising hydrolysates of tetraethyl ortho silicate(TEOS) and methyl trimethoxy silane (MTMS) were adjusted to obtainliquid compounds each containing the compounds by 12% by weight in termsof SiO₂ respectively (Example of coating liquids {circle around (3)}).The requirements for preparation of the liquid composition (coatingliquid for forming a coating film) are as shown in Table 1.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon wafer substrate (semiconductorsubstrate) with the size of 8×8 square inches using the spin coat methodunder the same conditions as those employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrates (Example substrate {circle around (3)}) wascooled down to a temperature around the room temperature, and taken outfrom the system.

The film thickness of the silica-based coating film formed on asubstrate obtained as described above was about 500 nm.

Then, like in Example 1, the silica-based coating film formed on asubstrate was measured for the specific dielectric constant, a changerate in moisture absorption by the coating film before and afterirradiation of an oxygen plasma, film strength, surface roughness, poredistribution (average diameter of pores and volume percentage ofmicropores each with the diameter of 2 nm or below against the totalvolume), and X-ray diffraction peaks (determined for an crystallinecoating film or an amorphous coating film). The result is shown in Table5.

EXAMPLE 4

300 g cation exchange resin powder (WK-40, produced by MitsubishiChemicals Corp.) was added to 1 kg aqueous solution containingtetrabutyl ammonium hydroxide (TBAOH, produced by Lion Corp.) by 40% byweight, and the resultant mixture solution was agitated for one hour atthe room temperature and at a rotating speed of 200 rpm, and then theadded cation exchange resin powder was removed by filtration. Then 2100g anion exchange resin powder (SAT-10, produced by Mitsubishi ChemicalsCorp.) was added to the mixture solution, and the resultant mixturesolution was agitated for one hour at the room temperature and at arotating speed of 200 rpm, and then the added anion exchange resinpowder was removed by filtration.

Like in the case of tetrapropyl ammonium hydroxide (TPAOH) described inExample 1, ultra pure water was added to this aqueous solution to adjustthe concentration to 10% by weight, and quantities of compounds ofalkali metal elements such as sodium (Na) and potassium (K) contained asimpurities in the aqueous solution as well as of compounds of halogengroup elements such as bromine (Br) and chlorine (Cl) contained asimpurities in the aqueous solution, were measured respectively by theatomic absorption spectrometry method (AAS method) and the ionchromatography method.

Further ultra pure water was added to an aqueous solution of thetetrabutyl ammonium hydroxide (commercially procurable) not having beensubjected to the ion exchange processes described above to adjust theconcentration to 10% by weight, and contents of impurities containedtherein were measured as described above.

As a result, contents of the impurities contained in the aqueoussolution not having been subjected to the ion exchange processes were 50ppm by weight for sodium, 3000 ppm by weight for potassium, 2500 ppm byweight for bromine, and 14 ppm by weight for chlorine as converted toeach element respectively, but those of the impurities contained in theaqueous solution having been subjected to the ion exchange processeswere 10 ppb by weight or below for sodium (lower limit for detection),10 ppb by weight or below for potassium (lower limit for detection), 1ppm by weight or below for bromine, and 1 ppm by weight or below forchlorine as converted to each element respectively.

Then, 85.7 g tetraethyl ortho silicate (TEOS, produced by Tama ChemicalIndustry Corp.), 127.3 g methyl trimethoxy silane (MTMS, produced byShin'etsu Kagaku K. K), and 406.4 g ethanol with the concentration of99.5% by weight (ETOH, produced Wako Jun'yaku K. K) were mixed, and themixture solution was kept at a temperature of 20° C. and agitated for 30minutes at a rotating speed of 150 rpm.

Then 380.6 g of highly purified aqueous solution of tetrapropyl ammoniumhydroxide (containing TBAOH by 10% by weight) was added into the mixturesolution over 10 minutes, and the resultant mixture solution was kept ata temperature of 20° C. and agitated for 1 hour at a rotating speed of150 rpm. Then the mixture solution was heated to a temperature of 50° C.and agitated keeping the temperature at the level above for 20 hours ata rotating speed of 200 rpm to hydrolyze the components for forming asilica-based coating film (TEOS and MTMS).

Then, like in Example 1, the mixture solution containing hydrolysates ofthe components for forming a silica-based coating film was subjected tothe solvent substitution process for substituting ethanol (organicsolvent) contained therein with propylene glycol monopropyl ether (PGP,produced by Nihon Emulsion Co.) using a rotary evaporator, and thenconcentrations of silicon compounds comprising hydrolysates oftetraethyl ortho silicate (TEOS) and methyltrimethoxy silane (MTMS) wereadjusted to obtain liquid compositions each containing the compounds by12% by weight in terms of SiO₂ (Examples of coating liquid {circlearound (4)}). The requirements for preparation of the liquidcompositions (coating liquid for forming a coating film) are as shown inTable 1.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon wafer substrate (semiconductorsubstrate) with the size of 8×8 square inches using the spin coat methodunder the same conditions as those employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrates (Example substrate {circle around (4)}) wascooled down to a temperature around the room temperature, and taken outfrom the system.

The film thickness of the silica-based coating film formed on asubstrate obtained as described above was about 500 nm.

Then, like in Example 1, the silica-based coating film formed on asubstrate was measured for the specific dielectric constant, a changerate in moisture absorption by the coating film before and afterirradiation of an oxygen plasma, film strength, surface roughness, poredistribution (average diameter of pores and volume percentage ofmicropores each with the diameter of 2 nm or below against the totalvolume), and X-ray diffraction peaks (determined for an crystallinecoating film or an amorphous coating film). The result is shown in Table5.

EXAMPLE 5

85.7 g tetraethyl ortho silicate (TEOS produced by Tama ChemicalIndustry Corp.) and 146.6 g ethanol with the concentration of 99.5% byweight (ETOH, produced Wako Jun'yaku K. K) were mixed with each other,and the mixture solution was kept at a temperature of 20° C. andagitated for 30 minutes at a rotating speed of 150 rpm. Then 89.5 gaqueous solution of the highly purified tetrapropyl ammonium hydroxide(containing TPAOH by 10% by weight) was added into the mixture solutionover 10 minutes, and agitated for 2 hours at a temperature of 20° C. andat a rotating speed of 150 rpm. Then the mixture solution was heated to50° C. and agitated at the temperature and at a rotating speed of 200rpm to hydrolyze the tetraethyl ortho silicate.

Then an aqueous solution prepared by mixing 127.3 g methyl trimethoxysilane (MTMS, produced by Shin'etsu Chemical Industry Corp.) with 342.1g ethanol with the concentration of 99.5% by weight (ETOH, produced byWako Jun'yaku K. K) was mixed in the mixture solution described above,and the resultant mixture solution was kept at a temperature of 20° C.and agitated for 10 minutes at a rotating speed of 150 rpm.

Then 208.8 g of highly purified aqueous solution of tetrapropyl ammoniumhydroxide (containing TPAOH by 10% by weight) was added into the mixturesolution over 10 minutes, and the resultant mixture solution was kept ata temperature of 20° C. and agitated for 1 hour at a rotating speed of150 rpm. Then the mixture solution was heated to a temperature of 50° C.and agitated keeping the temperature at the level above for 25 hours ata rotating speed of 200 rpm to hydrolyze methyl trimethoxy silane (MTMS)and the components to be hydrolyzed (partial hydrolysate of tetraethylortho silicate and the like).

Then, like in Example 1, the mixture solution containing hydrolysates ofthe components for forming a silica-based coating film was subjected tothe solvent substitution process for substituting ethanol in the mixturesolution with propylene glycol monopropyl ether (PGP) using a rotaryevaporator, and then concentration of a silicon compound comprisinghydrolysates of tetraethyl ortho silicate (TEOS) and methyl trimethoxysilane (MTMS) was adjusted to obtain liquid compositions containing thecompound by 12% by weight in terms of SiO₂ (Example of coating liquid{circle around (5)}). The requirements for preparation of the liquidcompositions (coating liquids for forming a coating film) are as shownin Table 1.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon wafer substrate (semiconductorsubstrate) with the size of 8×8 square inches using the spin coat methodunder the same conditions as those employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrates (Example substrate {circle around (5)}) wascooled down to a temperature around the room temperature, and taken outfrom the system.

The film thickness of the silica-based coating film formed on asubstrate obtained as described above was about 500 nm.

Then, like in Example 1, the silica-based coating film formed on asubstrate was measured for the specific dielectric constant, a changerate in moisture absorption by the coating film before and afterirradiation of an oxygen plasma, film strength, surface roughness, poredistribution (average diameter of pores and volume percentage ofmicropores each with the diameter of 2 nm or below against the totalvolume), and X-ray diffraction peaks (determined for an crystallinecoating film or an amorphous coating film). The result is shown in Table5.

EXAMPLE 6

85.7 g tetraethyl ortho silicate (TEOS, produced by Tama ChemicalIndustry Corp.) and 146.6 g ethanol with the concentration of 99.5% byweight (ETOH, produced Wako Jun'yaku K. K) were mixed with each other,and the mixture solution was kept at a temperature of 20° C. andagitated for 30 minutes at a rotating speed of 150 rpm.

Then 89.5 g aqueous solution of the highly purified tetrapropyl ammoniumhydroxide (containing TPAOH by 10% by weight) was added into the mixturesolution over 10 minutes, and agitated for 5 hours at a temperature of20° C. and at a rotating speed of 150 rpm. Then the mixture solution washeated to 50° C. and agitated at the temperature and at a rotating speedof 200 rpm for 40 hours to hydrolyze the tetraethyl ortho silicate.

Then 127.3 g methyl trimethoxy silane (MTMS, produced by Shin'etsuChemical Industry Corp.) and 342.1 g ethanol with the concentration of99.5% by weight (ETOH, produced by Wako Jun'yaku K. K) were mixed witheach other, and the resultant mixture solution was kept at a temperatureof 20° C. and agitated for 30 minutes at a rotating speed of 150 rpm.

Then 208.8 g of highly purified aqueous solution of tetrapropyl ammoniumhydroxide (containing TPAOH by 10% by weight) was added into the mixturesolution over 10 minutes, and the resultant mixture solution was kept ata temperature of 20° C. and agitated for 2 hours at a rotating speed of150 rpm. Then the mixture solution was heated to a temperature of 50° C.and agitated keeping the temperature at the level above for 5 hours at arotating speed of 200 rpm to hydrolyze methyl trimethoxy silane (MTMS)partially.

Next, the mixture solution was mixed, heated to 50° C. and agitated atthe temperature and at a rotating speed of 200 rpm for 20 hours tohydrolyze the components for forming a silica-based coating film (apartial hydrolysate of TEOS and MTMS).

Then, like in Example 1, the mixture solution containing hydrolysates ofthe components for forming a silica-based coating film was subjected tothe solvent substitution process for substituting ethanol in the mixturesolution with propylene glycol monopropyl ether (PGP) using a rotaryevaporator, and then concentration of a silicon compound comprisinghydrolysates of tetraethyl ortho silicate (TEOS) and methyl trimethoxysilane (MTMS) was adjusted to obtain liquid compositions containing thecompound by 12% by weight in terms of SiO₂ (Example of coating liquid{circle around (6)}). The requirements for preparation of the liquidcompositions (coating liquids for forming a coating film) are as shownin Table 1.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon wafer substrate (semiconductorsubstrate) with the size of 8×8 square inches using the spin coat methodunder the same conditions as those employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrates (Example substrate {circle around (6)}) wascooled down to a temperature around the room temperature, and taken outfrom the system.

The film thickness of the silica-based coating film formed on asubstrate obtained as described above was about 500 nm.

Then, like in Example 1, the silica-based coating film formed on asubstrate was measured for the specific dielectric constant, a changerate in moisture absorption by the coating film before and afterirradiation of an oxygen plasma, film strength, surface roughness, poredistribution (average diameter of pores and volume percentage ofmicropores each with the diameter of 2 nm or below against the totalvolume), and X-ray diffraction peaks (determined for an crystallinecoating film or an amorphous coating film). The result is shown in Table5.

EXAMPLE 7

77.1 g tetraethyl silicate (TEOS, produced by Tama Chemical IndustryCorp.), 114.5 g methyl trimethoxy silane (MTMS, produced by Shin'etsuChemical Industry Corp.), 80.0 g polysiloxane (PS, a product with theconcentration of 10% by weight in terms of SiO₂ prepared by the methoddescribed in Japanese Patent Laid-Open Publication No. 1997-315812), and430.1 g ethanol with the concentration of 99.5% by weight (ETOH,produced by Wako Jun'yaku Corp.) were mixed with each other, and themixture solution was kept at a temperature of 20° C. and agitated for 30minutes at a rotating speed of 150 rpm.

Then 298.3 g aqueous solution of the highly purified tetrapropylammonium hydroxide (including TPAOH by 10% by weight) was added into themixture solution over 10 minutes, and further was agitated at atemperature of 20° C. for one hour at a rotating speed of 150 rpm. Thenthe mixture solution was heated to 50° C., and agitated at thetemperature for 20 hours at a rotating speed of 200 rpm to hydrolyze thecomponents for forming a silica-based coating film (TEOS and MTMS).

Then, like in the case of Example 1, the mixture solution containinghydrolysates of the components for forming a silica-based coating filmwas subjected to the solvent substitution process for substitutingethanol in the mixture solution with propylene glycol monopropyl ether(PGP) using a rotary evaporator, and then concentration of a siliconcompound comprising hydrolysates of tetraethyl ortho silicate (TEOS) andmethyl trimethoxy silane (MTMS) was adjusted to obtain liquidcompositions containing the compound by 12% by weight in terms of SiO₂(Example of coating liquid {circle around (7)}). The requirements forpreparation of the liquid compositions (coating liquids for forming acoating film) are as shown in Table 1.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon wafer substrate (semiconductorsubstrate) with the size of 8×8 square inches using the spin coat methodunder the same conditions as those employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrates (Example substrate {circle around (7)}) wascooled down to a temperature around the room temperature, and taken outfrom the system.

The film thickness of the silica-based coating film formed on asubstrate obtained as described above was about 500 nm.

Then, like in Example 1, the silica-based coating film formed on asubstrate was measured for the specific dielectric constant, a changerate in moisture absorption by the coating film before and afterirradiation of an oxygen plasma, film strength, surface roughness, poredistribution (average diameter of pores and volume percentage ofmicropores each with the diameter of 2 nm or below against the totalvolume), and X-ray diffraction peaks (determined for an crystallinecoating film or an amorphous coating film). The result is shown in Table5.

EXAMPLE 8

5 ml of the coating liquid for forming a coating film (Example ofcoating liquid {circle around (2)}) was applied on a silicon wafersubstrate (semiconductor substrate) with the size of 8×8 square inchesusing the spin coat method under the same conditions as those employedin Example 1.

Then the substrates were placed on a hot plate of a single wafer system,and heated for 3 minutes in the air and at the temperature shown inTable 2. As the organic solvent (PGP) or the like evaporate during thisheating step, these evaporated materials were excluded from the system.

The substrates were kept placed on the hot plate of a single wafersystem, and the process environment was changed from the atmospheric airto nitrogen atmosphere, and then the substrates were subjected to thecuring step for 30 minutes at the temperatures as shown in Table 2. Thenthe substrates (Example substrates {circle around (8)}-1 to {circlearound (8)}-6) were cooled down to a temperature around the roomtemperature, and taken out from the system.

The film thickness of the silica-based coating film on a substrate asdescribed above was about 500 nm.

Then, the silica-based coating film formed on a substrate was measuredfor the specific dielectric constant, a change rate in moistureabsorption by the coating film before and after irradiation of an oxygenplasma, and others (such as appearance of the coating film, damages tothe semiconductor substrate). The result is shown in Table 6.

COMPARATIVE EXAMPLE 1

Tetraethyl ortho silicate (TEOS, produced by Tama Chemical IndustryCorp.), methyl trimethoxy silane (MTMS, produced by Shin'etsu ChemicalIndustry Corp.), and ethanol with the concentration of 99.5% by weight(ETOH, produced by Wako Jun'yaku Corp.) were mixed at the respectivelymixing ratios shown in Table 3, and the mixture solution was kept at atemperature of 20° C. and agitated for 30 minutes at a rotating speed of150 rpm.

An aqueous solution of the highly purified tetrapropyl ammoniumhydroxide (containing TPAOH by 10 weight %) was added into the mixturesolution above over 10 minutes at the mixing ratio shown in Table 1, andthe resultant mixture solution was agitated for one hours at 20° C. andat a rotating speed of 250 rpm. Then the mixture solution was heated to50° C. and agitated for 20 hours at the temperature and at a rotatingspeed of 250 rpm to hydrolyze the components for forming a silica-basedcoating film (TEOS and MTMS).

Then, like in the case of Example 1, the mixture solution containinghydrolysates of the components for forming a silica-based coating filmwas subjected to the solvent substitution process for substitutingethanol in the mixture solution to propylene glycol monopropyl ether(PGP) using a rotary evaporator, and then concentration of siliconcompounds comprising hydrolysates of tetraethyl ortho silicate (TEOS)and methyl trimethoxy silane (MTMS) were adjusted to obtain liquidcompositions containing the compound by 12% by weight in terms of SiO₂(Comparative examples of coating liquids {circle around (1)}-1 to{circle around (1)}-4). The requirements for preparation of the liquidcompositions (coating liquids each for forming a coating liquid) are asshown in Table 3.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon substrate with the size of 8×8square inches using the spin coat method under the same conditions asthose employed in Example 1.

Then, the substrates were subjected to the heating step as well as tothe curing step under the same conditions as those employed inExample 1. Further, the substrates (Comparative examples of substrates{circle around (1)}-1 to {circle around (1)}-4) were cooled down to atemperature around the room temperature, and taken out from the system.

The film thickness of the silica-based coating film formed on asubstrate as described above was about 500 nm.

Next, like in the case of Example 1, the silica-based coating filmformed on a substrate was measured for the specific dielectric constant(if the coating film would absorb moisture, the dielectric constant wasalso measured after left for one day in the air), a change rate inmoisture absorption by the coating liquid before and after irradiationof an oxygen plasma, film strength, surface roughness, pore distribution(average diameter of pores and volume percentage of micropores each withthe diameter of 2 nm or below against the total volume), and X-raydiffraction peaks (determined for an crystalline coating film or anamorphous coating film). The result is shown in Table 5.

COMPARATIVE EXAMPLE 2

85.7 g tetraethyl ortho silicate (TEOS, produced by Tama ChemicalIndustry Corp.), 127.3 g methyl trimethoxy silane (MTMS, produced byShin'etsu Chemical Industry Corp.), and 488.7 g ethanol with theconcentration of 5% by weight (ETOH, produced by Wako Jun'yaku Corp.)were mixed with each other, and the mixture solution was kept at atemperature of 20° C. and agitated for 30 minutes at a rotating speed of150 rpm.

298.3 g of an aqueous solution of the not highly purified tetrapropylammonium hydroxide (containing TPAOH by 10 weight %) was added into themixture solution above over 10 minutes, and the resultant mixturesolution was agitated for one hours at 20° C. and at a rotating speed of150 rpm. Then the mixture solution was heated to 50° C. and agitated for20 hours at the temperature and at a rotating speed of 200 rpm tohydrolyze the components for forming a silica-based coating film (TEOSand MTMS).

Then, like in the case of Example 1, the mixture solution containinghydrolysates of the components for forming a silica-based coating filmwas subjected to the solvent substitution process for substitutingethanol (organic solvent) in the mixture solution to propylene glycolmonopropyl ether (PGP) using a rotary evaporator, and then concentrationof silicon compounds comprising hydrolysates of tetraethyl orthosilicate (TEOS) and methyl trimethoxy silane (MTMS) were adjusted toobtain liquid compositions containing the compound by 12% by weight interms of SiO₂ (Comparative example of coating liquids {circle around(2)}). The requirements for preparation of the liquid compositions(coating liquids each for forming a coating liquid) are as shown inTable 3.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon substrate with the size of 8×8square inches using the spin coat method under the same conditions asthose employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrate (Comparative example of substrate {circle around(2)}) was cooled down to a temperature around the room temperature, andtaken out from the system.

The film thickness of the silica-based coating film formed on asubstrate as described above was about 500 nm.

Next, like in the case of Example 1, the silica-based coating filmformed on a substrate was measured for the specific dielectric constant(if it is observed that the coating film would absorb moisture, thedielectric constant was also measured after left for one day in theair), a change rate in moisture absorption by the coating liquid beforeand after irradiation of an oxygen plasma, film strength, surfaceroughness, pore distribution (average diameter of pores and volumepercentage of micropores each with the diameter of 2 nm or below againstthe total volume), and X-ray diffraction peaks (determined for ancrystalline coating film or an amorphous coating film). The result isshown in Table 5.

COMPARATIVE EXAMPLE 3

By employing the same method as the known ones (described, for instance,in non-patent document 1, patent document 6), 285.7 g tetraethyl orthosilicate (TEOS, produced by Tama Chemical Industry Corp.), and 574.3 gethanol with the concentration of 5% by weight (ETOH, produced by WakoJun'yaku Corp.) were mixed with each other, and the mixture solution waskept at a temperature of 20° C. and was agitated for 30 minutes at arotating speed of 150 rpm.

140.0 g of an aqueous solution of the not highly purified tetrapropylammonium hydroxide (containing TPAOH by 21.3% by weight) was added intothe mixture solution above over 2 hours, and the resultant mixturesolution was agitated for three days at 20° C. and at a rotating speedof 150 rpm. Then the mixture solution was heated to 80° C. and agitatedfor 3 days at the temperature and at a rotating speed of 200 rpm tohydrolyze the tetraethyl ortho silicate (TEOS).

As a mixture solution with white turbidity was obtained, and thissolution was processed for 20 minutes in a centrifugal separator at arotating speed of 2000 rpm to remove large particles.

Then, like in the case of Example 1, the mixture solution was subjectedto the solvent substitution process for substituting ethanol containedin this mixture solution with propylene glycol monopropyl ether (PGP )using a rotary evaporator, and then the concentration of siliconcompounds comprising a hydrolysate of tetraethyl ortho silicate. (TEOS)to obtain a liquid composition containing the compound by 12% by weightin terms of SiO₂ (Comparative example of coating liquid {circle around(3)}). The requirements for preparation of this liquid composition(coating liquid for forming a coating film) is as shown in Table 3.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon substrate with the size of 8×8square inches using the spin coat method under the same conditions asthose employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrate (Comparative example of substrate {circle around(3)}) was cooled down to a temperature around the room temperature, andtaken out from the system.

The film thickness of the silica-based coating film formed on asubstrate as described above was about 500 nm.

Next, like in the case of Example 1, the silica-based coating filmformed on a substrate was measured for the specific dielectric constant(if it is observed that the coating film would absorb moisture, thedielectric constant was also measured after left for one day in theair), a change rate in moisture absorption by the coating liquid beforeand after irradiation of an oxygen plasma, film strength, surfaceroughness, pore distribution (average diameter of pores and volumepercentage of micropores each with the diameter of 2 nm or below againstthe total volume), and X-ray diffraction peaks (determined for ancrystalline coating film or an amorphous coating film).

The result is shown in Table 5. Further a result of X-ray diffraction ofthe silica-based coating film formed on the substrate is shown in FIG.2.

COMPARATIVE EXAMPLE 4

Like in the case of Comparative Example 3, 285.7 g tetraethyl orthosilicate (TEOS, produced by Tama Chemical Industry Corp.), and 574.3 gethanol with the concentration of 99.5% by weight (ETOH, produced byWako Jun'yaku Corp.) were mixed with each other, and the mixturesolution was kept at a temperature of 20° C. and was agitated for 30minutes at a rotating speed of 150 rpm.

140.0 g of an aqueous solution of the highly purified tetrapropylammonium hydroxide (containing TPAOH by 21.3% by weight) was added intothe mixture solution above over 2 hours, and the resultant mixturesolution was agitated for three days at 20° C. and at a rotating speedof 150 rpm. Then the mixture solution was heated to 80° C. and agitatedfor 3 days at the temperature and at a rotating speed of 200 rpm tohydrolyze the tetraethyl ortho silicate (TEOS). Different from the caseof Comparative Example 3, a product with white turbidity was notobtained.

Then, in the case of Example 1, the mixture solution was subjected tothe solvent substitution process for substituting ethanol contained inthis mixture solution with propylene glycol monopropyl ether (PGP) usinga rotary evaporator, and then the concentration of silicon compoundscomprising a hydrolysate of tetraethyl ortho silicate (TEOS) to obtain aliquid composition (Comparative example of coating liquid {circle around(4)}) containing the compound by 12% by weight in terms of SiO₂. Therequirements for preparation of this liquid composition (coating liquidfor forming a coating film) is as shown in Table 3.

5 ml of the coating liquid for forming a coating film obtained asdescribed above was applied on a silicon substrate with the size of 8×8square inches using the spin coat method under the same conditions asthose employed in Example 1.

Then the substrate was subjected to the heating step as well as to thecuring step under the same conditions as those employed in Example 1.Further the substrate (Comparative example of substrate {circle around(4)}) was cooled down to a temperature around the room temperature, andtaken out from the system.

The film thickness of the silica-based coating film formed on asubstrate as described above was about 500 nm.

Next, like in the case of Example 1, the silica-based coating filmformed on a substrate was measured for the specific dielectric constant(if it is observed that the coating film would absorb moisture, thedielectric constant was also measured after left for one day in theair), a change rate in moisture absorption by the coating liquid beforeand after irradiation of an oxygen plasma, film strength, surfaceroughness, pore distribution (average diameter of pores and volumepercentage of micropores each with the diameter of 2 nm or below againstthe total volume), and X-ray diffraction peaks (determined for ancrystalline coating film or an amorphous coating film). The result isshown in Table 5.

COMPARATIVE EXAMPLE 5

5 ml of the coating liquid for forming a coating film (Example ofcoating liquids {circle around (2)}) obtained Example 2 was applied on asilicon substrate with the size of 8×8 square inches using the spin coatmethod under the same conditions as those employed in Example 1.

Then the substrates were placed on a hot plate of a single wafer systemand were subjected to the heating step for 3 minutes in the atmosphericair and at the temperatures shown in Table 4. During this heating step,the organic solvents (PGP) and the like contained in the coating filmevaporate, and the evaporated materials were taken out from the system.

Further the substrates were kept placed on the hot plate of a singlewafer system, and the processing environment was changed from theatmospheric air to nitrogen environment, and the substrates weresubjected to the curing step for 30 minutes at the temperature shown inTable 4. Then the substrates (Comparative examples of substrates {circlearound (5)}-1 to {circle around (5)}-6) were cooled down to atemperature around the room temperature, and taken out from the system.

The film thickness of the silica-based coating film formed on asubstrate was about 500 nm.

Next, the silica-based coating film formed on a substrate was measuredfor the specific dielectric constant, a change rate in moistureabsorption by the coating liquid before and after irradiation of anoxygen plasma, film strength, and other matters (appearance of eachcoating film or damages to the semiconductor substrate). The result isshown in Table 6.

As clearly understood from the result of measurement shown in Table 5,when a coating liquid is formed on a substrate with the coating liquidfor forming a coating film according to the present invention, it ispossible to form an amorphous silica-based coating film with thespecific dielectric constant of 2.5 or below and the film strengthexpressed by Young's modulus of 6.0 GPa or more.

Further as this silica-based coating film itself has the excellenthydrophobic property (moisture absorption resisting characteristics), sothat the coating film can preserve the hydrophobic property for a longtime without being subjected to a surface processing such as silylationrecommended in the known examples (non-patent document 1, patentdocument 6 and the like), and as a result degradation of dielectricconstant never occurs. In addition, it has been found that the amorphoussilica-based coating film has not only more excellent hydrophobicproperty as compared to a crystalline coating liquid such as a zeoliticcoating film, but also the surface smoothness with irregularities of 1nm or less. The average diameter of pores in the silica-based coatingfilm is 3 nm or less, and volume percentage of micropores each with thediameter of 2 nm or less was 70% or more.

Further, it has been found that, when the coating liquid for forming acoating film is to be prepared, it is necessary to remove impuritiescomprising compounds of alkali metal elements such as sodium (Na) andpotassium (K) contained in the tetraalkyl ammonium hydroxide (TAAOH)used in the preparation process to the revel described above.

Further it has been found that a desired silica-based coating film cannot always be obtained unless, in preparation of the coating liquid forforming a coating film, a molar ratio of tetraalkyl ortho silicate(TAOS) such as TEOS versus alkoxysilane (AS) such as MTMS (TAOS/AS) inthe range from 6/4 to 2/8 in terms of SiO₂ and also adjusting a molarratio (TAAOH/(TAOS+AS)) of tetraalkyl ammonium hydroxide (TAAOH) versuscomponents for forming a coating film (TAOS+AS) in the range from 1/10to 7/10 also in terms of SiO₂. Further it has been found that, even whenpolysiloxane (PS) is contained at the mixing ratio described above, thespecific dielectric constant and strength of the coating film are notaffected.

In contrast, when a coating film is formed on a substrate using any ofthe coating liquids each for forming a coating film described inComparative Examples, although a portion of the properties describedabove could be obtained, all of the properties or effects could notalways be shown. Namely the coating liquids described in ComparativeExamples can not satisfy the recent requirements from semiconductormanufacturers.

With Comparative examples of substrates {circle around (3)} and {circlearound (4)} (using the coating liquids each for forming a coating filmprepared without using alkoxysilane (AS) such as MTMS), it is possibleto obtain a coating film having the Young's modulus of 6.0 GPa or more,but the surface roughness is substantially remarkable, and it has beenfound that, when the substrate is left in the atmospheric air for oneday, the specific dielectric constant rapidly increased from 2.3 to 3.0.Because of the characteristics, it is necessary to subject a surface ofthe substrate to polishing and silylation for practically using thesubstrate. With Comparative example of substrate {circle around (2)}(using the coating liquids each for forming a coating film prepared withthe tetrapropyl ammonium hydroxide (TPAOH) containing impurities and nothaving been highly purified), although a silica-based coating filmhaving a desired dielectric constant and strength can be obtained, aportion of the coating film is crystallized, and the surface roughnessof 1 nm or more was observed.

Further, with Comparative examples of substrates {circle around (1)}-1and {circle around (1)}-3, the required film strength can not beobtained, and with Comparative examples of substrates {circle around(1)}-2 and {circle around (1)}-4, the desired dielectric constant cannot be obtained. In addition, with Comparative example of substrate{circle around (1)}-2, the moisture absorption rate increases, and whenthe substrate is left for one day in the atmospheric air, the desiredspecific dielectric constant will be lost.

Further, as clearly understood from the measurement result shown inTable 6, when a coating film is formed on a substrate using thepreparation method containing heating step and curing step according tothe present invention, it is possible not only to obtain an amorphoussilica-based coating film having the specific dielectric constant of 2.5or below and high film strength expressed by Young's modulus of 6.0 GPaor more, but also to obtain a silica-based coating film excellent inadhesiveness to a surface of a substrate on which a coating film is tobe formed, the chemical resistance such as alkali resistance, andcracking resistance, and further excellent in oxygen plasma resistanceand also in adaptability to various processes such as etching.

Further, when the heating step and the curing step are carried out underthe operating conditions as specified in the present invention,non-uniformity in the thickness of a coating film formed as describedabove is not generated, and no damage is given to aluminum wiring orcopper wiring on a semiconductor substrate.

In contrast, when a coating film is formed on a substrate by using anyof the preparation methods containing heating step and curing stepdescribed in the Comparative examples, although a portion of thecharacteristics described above can be obtained, all of the propertiesor effects can not always be shown.

Further it has been found that, when the heating step is carried out ata temperature of under 80° C. or of over 350° C., non-uniformity inthickness of the coating film occurs. Also it has been found that, whenthe curing step is carried out at a temperature of under 350° C., amoisture absorption rate in the coating film increases, and also that,when the process is carried out at a temperature of over 450° C.,damages are easily given to a semiconductor substrate.

As clearly understood from the descriptions above, it can be said thatthe method is most suited to formation of a low dielectric constantamorphous silica-based coating film having a high film strength andhydrophobic property (e.g., the moisture absorption resistingcharacteristics) with its surface being smooth or even. TABLE 1 Examplesof Coating Liquids Added Mixed amount Molar ratio amount Mixed Mixed ofMolar ratio of of amount amount TPAOH of TAAOH/ TEOS of of or TBAOHTAOS/AS (TAOS + AS) Example or TMOS MTES ETOH solution (in terms of (interms of No. (g) (g) (g) (g) SiO₂) SiO₂) {circle around (1)}-1 57.1145.5 499.1 298.3 2/8 1.1/10 {circle around (1)}-2 85.7 127.3 488.7298.3 3/7 1.1/10 {circle around (1)}-3 85.7 127.3 488.7 1084.7  3/7  4/10 {circle around (1)}-4 53.6 79.5 273.7 593.2 3/7   7/10 {circlearound (1)}-5 142.9 90.9 467.9 298.3 5/5 1.1/10 {circle around (1)}-6142.9 90.9 467.9 1084.7  5/5   4/10 {circle around (1)}-7 89.3 56.8260.7 593.2 5/5   7/10 {circle around (1)}-8 171.4 72.7 457.6 298.3 6/41.1/10 {circle around (2)} 60.8 127.3 513.6 298.3 3/7 1.1/10 (TMOS){circle around (3)} 85.7 166.2 449.8 298.3 3/7 1.1/10 (MTES) {circlearound (4)} 85.7 127.3 406.4 380.6 3/7 1.1/10 (TBAOH) {circle around(5)} 85.7 127.3 488.7 298.3 3/7 1.1/10 (Total) (Total) {circle around(6)} 85.7 127.3 488.7 298.3 3/7 1.1/10 (Total) (Total) {circle around(7)} 77.1 114.5 430.1 298.3 3/7 1.1/10 (containing PS by 10% by weight)

TABLE 2 Conditions for Heating step and Curing step in Examples Heatingstep Curing step Processing Processing Processing Processing Exampletemperature Processing time temperature Processing time No. (° C.)atmosphere (minute) (° C.) atmosphere (minute) {circle around (8)}-1 100Air 3 380 Nitrogen 30 Gas {circle around (8)}-2 150 Air 3 380 Nitrogen30 Gas {circle around (8)}-3 150 Air 3 400 Nitrogen 30 Gas {circlearound (8)}-4 150 Air 3 450 Nitrogen 30 Gas {circle around (8)}-5 250Air 3 400 Nitrogen 30 Gas {circle around (8)}-6 300 Air 3 400 Nitrogen30 Gas

TABLE 3 Comparative Examples of Coating Liquids Added Molar ratio MixedMixed Mixed amount Molar ratio of amount amount amount of of TAAOH/Comparative of of of TPAOH TAOS/MTMS (TAOS + AS) Example TEOS MTMS ETOHsolution (in terms of (in terms of No. (g) (g) (g) (g) SiO₂) SiO₂){circle around (1)}-1 28.6 163.6 509.5 298.3 1/9 1.1/10 {circle around(1)}-2 200.0 54.5 447.2 298.3 7/3 1.1/10 {circle around (1)}-3 85.7127.3 597.2 189.8 3/7 0.7/10 {circle around (1)}-4 53.6 79.5 189.0 677.93/7   8/10 {circle around (2)} 85.7 127.3 488.7 298.3 3/7 1.1/10(containing impurities) {circle around (3)} 285.7 0 574.3 140.0 10/0 1.1/10 (containing impurities) {circle around (4)} 285.7 0 574.3 140.010/0  1.1/10

TABLE 4 Conditions for Heating step and Curing step in ComparativeExamples Heating step Curing step Comparative Processing ProcessingProcessing Processing Example temperature Processing time temperatureProcessing time No. (° C.) atmosphere (minute) (° C.) atmosphere(minute) {circle around (5)}-1 70 Air 3 400 Nitrogen 30 gas {circlearound (5)}-2 360 Air 3 400 Nitrogen 30 gas {circle around (5)}-3 150Air 3 340 Nitrogen 30 gas {circle around (5)}-4 150 Air 3 460 Nitrogen30 gas {circle around (5)}-5 250 Air 3 340 Nitrogen 30 gas {circlearound (5)}-6 250 Air 3 460 Nitrogen 30 gas

TABLE 5 Measurement Results (A) of Coating Films Average Specific ChangeFilm pore dielectric of moisture strength Surface diameter constantabsorption of roughness (nm) and X ray (Value in after coating of volumediffraction parenthesis oxygen film coating percentage peaks Comparativeis that left plasma (Young's film of (crystalline Example Example in theair irradiation modulus)) (Rms) micropores or No. No. for 1 day) (Yes orNo) (GPa) (nm) (%) amorphous) {circle around (1)}-1 2.2 No 6 0.7 2.7/72Amorphous {circle around (1)}-2 2.3 No 9 0.6 2.5/75 Amorphous {circlearound (1)}-3 2.3 No 9 0.5 2.4/75 Amorphous {circle around (1)}-4 2.3 No8 0.6 2.3/76 Amorphous {circle around (1)}-5 2.4 No 9 0.6 2.1/76Amorphous {circle around (1)}-6 2.4 No 9 0.6 2.1/75 Amorphous {circlearound (1)}-7 2.4 No 8 0.6 2.6/78 Amorphous {circle around (1)}-8 2.4 No10 0.7 2.2/77 Amorphous {circle around (2)} 2.3 No 10 0.6 2.3/77Amorphous {circle around (3)} 2.3 No 9 0.7 2.4/76 Amorphous {circlearound (4)} 2.3 No 8 0.6 2.4/77 Amorphous {circle around (5)} 2.4 No 60.6 2.3/75 Amorphous {circle around (6)} 2.4 No 6 0.6 2.3/76 Amorphous{circle around (7)} 2.4 No 7 0.6 2.4/75 Amorphous {circle around (1)}-12.9 No 2 2.5 2.8/71 Amorphous {circle around (1)}-2 2.6 Yes 10 0.72.2/78 Amorphous (3.0) (Moisture absorption increased) {circle around(1)}-3 2.4 No 5 0.6 2.3/73 Amorphous {circle around (1)}-4 2.6 No 7 0.62.5/71 Amorphous {circle around (2)} 2.3 No 12 2.7 2.4/75 Partiallycrystalline {circle around (3)} 2.3 Yes 11 4.1 2.2/74 Crystalline (3.0)(Moisture absorption increased) {circle around (4)} 2.3 Yes 9 3.1 2.4/72Amorphous (3.0) (Moisture absorption increased)

TABLE 6 Measurement Results (B) of Coating Films Change of Specificdielectric moisture Film strength Constant absorption of Appearance(Value in after coating film of Comparative parenthesis oxygen plasma(Young's coating film Example Example is that left in the irradiationmodulus) and other No. No. air for 1 day) (Yes or No) (GPa) observations{circle around (8)}-1 2.3 No 5 Good {circle around (8)}-2 2.3 No 5 Good{circle around (8)}-3 2.2 No 6 Good {circle around (8)}-4 2.1 No 8 Good{circle around (8)}-5 2.2 No 6 Good {circle around (8)}-6 2.2 No 6 Good{circle around (5)}-1 2.3 No 7 Uneven film thickness {circle around(5)}-2 2.3 No 10 Uneven film thickness {circle around (5)}-3 2.5 Yes(Moisture 6 Good (3.0) absorption increased) {circle around (5)}-4 2.7Yes (Moisture 11 Damages on (3.0) absorption substrate increased){circle around (5)}-5 2.5 Yes (Moisture 6 Good (3.0) absorptionincreased) {circle around (5)}-6 2.7 Yes (Moisture 11 Damages on (3.0)absorption substrate increased)

1. A method of forming an amorphous silica-based coating film with a lowdielectric constant having a high film strength and excellenthydrophobic property and capable of ensuring smoothness of a surfacecoated therewith on a substrate comprising the steps of: (a) preparing aliquid composition containing a silicon compound obtained by hydrolyzingtetraalkyl ortho silicate (TAOS) and alkoxysilane (AS) expressed by thefollowing general formula (I) in the presence of tetraalkyl ammoniumhydroxide (TAAOH):X_(n)Si(OR)_(4-n)   (I) wherein X indicates a hydrogen atom, a fluorineatom, or an alkyl group, a fluorine-substituted alkyl group, an arylgroup or a vinyl group each having 1 to 8 carbon atoms; R indicates ahydrogen atom, or an alkyl group, an aryl group or a vinyl group eachhaving 1 to 8 carbon atoms; and n is an integral number from 0 to
 3. (b)applying the liquid composition on a substrate; (c) heating thesubstrate at a temperature in a range from 80 to 350° C.; and (d) curingthe substrate at a temperature in a range from 350 to 450° C.
 2. Amethod of forming an amorphous silica-based coating film with a lowdielectric constant having a high film strength and excellenthydrophobic property and capable of ensuring smoothness of a surfacecoated therewith on a substrate comprising the steps of: (a) preparing aliquid composition containing a silicon compound obtained by hydrolyzingor partially hydrolyzing tetraalkyl ortho silicate (TAOS) in thepresence of tetraalkyl ammonium hydroxide (TAAOH), mixing the reactionproduct with the alkoxysilane (AS) expressed by the general formula (I)above or a hydrolysate or a partial hydrolysate thereof, and furtherhydrolyzing all or a portion of the mixture according to the necessity;(b) applying the liquid composition on a substrate; (c) heating thesubstrate at a temperature in a range from 80 to 350° C.; and (d) curingthe substrate at a temperature in a range from 350 to 450° C.
 3. Themethod of forming an amorphous silica-based coating film with a lowdielectric constant according to claim 1, wherein said tetraalkyl orthosilicate (TAOS) used in the preparing step (a) is tetraethyl orthosilicate (TEOS), tetramethyl ortho silicate (TMOS) or a mixture thereof.4. The method of forming an amorphous silica-based coating film with alow dielectric constant according to claim 1, wherein said alkoxysilane(AS) used in the preparing step (a) is methytrimethoxy silane (MTMS),methyltriethoxy silane (MTES) or a mixture thereof.
 5. The method offorming an amorphous silica-based coating film with a low dielectricconstant according to claim 1, wherein said tetraalkyl ammoniumhydroxide (TAAOH) used in the preparing step (a) is tetrapropyl ammoniumhydroxide (TPAOH), tetrabutyl ammonium hydroxide (TBAOH) or a mixturethereof.
 6. The method of forming an amorphous silica-based coating filmwith a low dielectric constant according to claim 1, wherein a contentof impurities comprising compounds of alkali metal elements such assodium (Na) and potassium (K) contained in said tetraalkyl ammoniumhydroxide (TAAOH) used in the preparing step (a) is 50 ppb by weight orbelow on respective element bases.
 7. The method of forming an amorphoussilica-based coating film with a low dielectric constant according toclaim 1, wherein a content of impurities comprising compounds of halogengroup elements such as bromine (Br) and chlorine (Cl) contained in saidtetraalkyl ammonium hydroxide (TAAOH) used in the preparing step (a) is1 ppm by weight or less on respective element bases.
 8. The method offorming an amorphous silica-based coating film with a low dielectricconstant according to claim 1, wherein a molar ratio (TAOS/AS) of saidtetraalkyl ortho silicate (TAOS) and said alkoxysilane (AS) used in thepreparing step (a) is in a range from 6/4 to 2/8 in terms of SO2.
 9. Themethod of forming an amorphous silica-based coating film with a lowdielectric constant according to claim 1, wherein a molar ratio(TAAOH)/(TAOS+AS) of said tetraalkyl ammonium hydroxide (TAAOH) and thecomponents for forming a silica-based coating film (TAOS+AS) used in thepreparing step (a) is in a range from 1/10 to 7/10 in terms of SO2. 10.The method of forming an amorphous silica-based coating film with a lowdielectric constant according to claim 1, wherein the operation used inthe applying step (b) is executed with a spin coat method.
 11. Themethod of forming an amorphous silica-based coating film with a lowdielectric constant according to claim 1, wherein the operation used inthe heating step (c) is executed for 1 to 10 minutes in the atmosphereof nitrogen or air.
 12. The method of forming an amorphous silica-basedcoating film with a low dielectric constant according to claim 1,wherein the operation used in the curing step (d) is executed for 5 to90 minutes in the atmosphere of nitrogen.
 13. An amorphous silica-basedcoating film with a low dielectric constant, wherein the obtainedcoating film using the method according to claim 1 has a specificdielectric constant of 2.5 or below and a film strength of Young'smodulus of 6.0 GPa or more.
 14. The amorphous silica-based coating filmwith a low dielectric constant according to claim to 13, wherein saidcoating film contains pores having an average diameter of 3 nm or belowand also with volume percentage of micropores each with the diameter of2 nm or below of 70% or more.
 15. The amorphous silica-based coatingfilm with a low dielectric constant according to claim 13, wherein saidcoating film has a smooth surface with surface roughness (Rms) of 1 nmor below.
 16. The amorphous silica-based coating film with a lowdielectric constant according to claim 13, wherein said coating film isan amorphous silica-based coating film not having any X-ray diffractionpeak specific to a MFI crystal structure.
 17. The amorphous silica-basedcoating film with a low dielectric constant according to claim 13,wherein said coating film is an inter-layer insulation film or aninter-metal insulation film formed on a semiconductor substrate.